I need help in buying my first bike

StickSpike

I want to buy my first ever bike and I would really appreciate some advice.I'll tell you what I think I want ,let me know what is unrealistic.

First of all,the purpose of the bike will be commuting. I will buy it second hand and my budget is around 700 dollars. Here in Romania there are plenty of really good deals on most stuff and I'm willing to wait quite a few months until I find a great one.
I would love for the bike to be under 12kg or so.

1. I understand that the geometry of the bike,reach and stack in particular, is very important. I totally understand how those work and I am not necessarily looking for a very up-right position,since I am quite flexible.
2. Frame->I guess my options for the frame are steel or aluminium. I'm not sure on what to choose. For this price,is steel usually that much heavier than aluminum? I am leaning towards steel because I assume it's more durable,let me know if I'm wrong. I know that there are plenty of different types of steel and heard a lot of good things about chromoly 4130 and some reynolds,but I am clearly not well informed in this subject,nor the price for that matter. Other than the type of steel used and of course,the geometry, is there anything else I need to look for in a steel frame? what about an aluminum one?
3. Fenders+racks->another point to make is that I really want full-coverage fenders and a rear bike rack with panniers for carrying groceries and stuff. I know this is probably a really dumb question,but since this is my first bike and I'm buying it online,how can I tell if it can take fenders and racks? Should I be looking for specific holes or something? For the rack,I think they are called seatstay eylets and dropout eylets. Am I correct? is that everything that's needed to take a regular rack? How about the fenders? I really want wide tyres and I don't know if it's even possible to get them paired with fenders. Is there a way to tell how much clearance do I have for the tyres with fenders before I even buy it?
4. Fork->All I know is that I don't want any suspensions. I plan on running tyres as wide as I can and ride on the road for the most part,so I don't need suspensions. Would an aluminum fork be better than a steel one? I think steel would dampen more road vibrations,right? Is there anything else I have to look for?Maybe the design of it? I assume this is the part that dictates the maximum width of the tyre,so I need to check for that. Is there any other part of the bike that might not allow me get a bit wider tyres,other than the fork and the rim ?
5. Handlebars-> a wide compact dropped handlebar sounds the most appealing to me. Even though I would be in a lower,more aggresive position with dropped handlebars compared to flat,the multitude of hand positions still seems more comfortable to me. When choosing handlebars,is it all about the shape? From what I've read,aluminum handlebars are very durable and people don't really recommend going for steel ones,so aluminum it is?
6. wheels+tyres-> I guess 700c rims are the most common. As I said,I want as wide tyres as I can fit with fenders..so I need to check that the rims fit wider than 28mm,right? Since I am not the lightest of riders at 80kg and I also plan on carrying stuff around in panniers,I guess the number of spokes matter? Would it make my bike be able to carry much more stuff without a problem or does it not really matter for me? I don't really know anything about choosing my rims,so maybe you guys could help me out.
7. Crankset-> 50/34 Compact crankset seems great for me,especially as a beginner. There are some climbs where I live,but none of them are that steep. Should I be also looking in 52/36 cranksets if I find good deals? From what I know,I will be looking for either Shimano Tiagra or Shimano 105. I know that there are plenty of versions of them and that they upgrade them quite often. I think the tiagra used to be 9-speed and now it is 10-speed,for example. are there specific models of each that I should look for? Or better said,are there any models that I should stay away from? Are there any other good alternatives to shimano? SRAM tends to be a bit more expensive here in Romania,but in case I find a better deal,are the SRAM Rival or the SRAM Force good alternatives to the tiagra/105?
8. Cassettes-> well,I'm pretty sure that if I choose a 105 crankset,I have to get a shimano 105 11-speed cassette. And so it's the case for any crankset I think. I will be looking to get as wide range as possible on the tooth options. I think the widest range on 105 is 11-32,while the widest on tiagra is 11-34. Anything else I should look into?
9. Brakes-> I don't mind the extra weight of disc brakes and I think they require less maintanance and are also more effective in wet conditions,so I will probably go with disc brakes. I couldn't really find any clear recommendations for mechanical disc brakes,other than TRP Spyre and BB7. These seem fairly expensive. Is there really no other cheaper disc brakes that stand out from the others? I really don't need extremely good performance,I'm only commuting. But I don't want to have awful brakes either. I did however find plenty recommendations for hidraulic disk brakes. And they aren't even that expensive. What is the deal with them,should I look for hydraulics? I think the levers are much more expensive,right? So It probably wouldn't be worth it to search for a bike with hydraulic disk breakes in my budget.
10. Levers-> I honestly don't know anything about these. I guess it depends on the crankset you get? So does a tiagra crankset only work with tiagra levers? I also don't know if they have to be specifically made for disk brakes. Because I'm wondering wether there are different models that are good that I need to look for.
11. Derailleurs-> again,i don't know much about these. I assume you need suitable front and back derailleurs for the type of crankset/cassettes you have. So again,should I only look for tiagra derailleurs if I have tiagra crankset/cassette?
12. Other stuff-> Stem,seat post,pedals .. anything I need to look for? I get that stems can change the reach and stack of the bike and that the longer the stem,the better control you have. As for pedals,I want flat pedals because I want to be able to use the bike in regular shoes.

Is there anything else? Thank you so much by the way for helping me!! Truly appreciate it if you do.

yiannism

It will be much easier if you give us some options that you already look to get an idea for what you can get in Romania, and we can trust from there.

shirley_basso

Are you building the bike from scratch?

Buy something off the peg and learn as you go as you'll spend a lot of time researching for no discernible benefit as a beginner.