Doug Hartley racing bike i just bought, plus questions, please advise

Yesterday i bought a racing bike, its the first one ive ever owned, im 47 and have previously had mountain bikes etc. Im keen to have a bike that offers a bit more speed on the roads.

I paid £35 off Gumtree for it, see photos attached, its fairly light, around 10kg or so and not sure what the frame is, possibly a 105 or 531, or maybe neither of those. The bike needs work doing, the guy said its been in his shed for 5 years and hed bought it to do up but never got round to it.

Questions: i want to put flat handlebars on it and get rid of the drop handlebars, is that eays to do and what gears should i buy, would a shimano combination brake and gear thumb shifter be suitable? The rear gears are the simplex make, its a ten speed in total.

Front derailleur looks like its seen better days though may work with a bit of work and new gear cables.

Also, id like to put slightly wider tyres on it, at present its got 700x25, what is the next size up and would it fit the rims i have ? Im going to replace the seat to something newer/smaller, will it be a standard seat i need ?

Im guessing bike is from the 1980s

If anyone can advise and or post links to items i can buy off ebay at a good price used or new or decathlon etc such as suitable bars and gear shifters, tyres etc it would be much appreciated. Im out of work and funds are fairly limited though i can buy parts as i go, ideally reconditioned second hand parts would be good though maybe not for tyres.