Specialized Roubaix SL4 Upgrade

OP, you don't need to spend anything close to that amount of cash to improve the bike...so start by spending less. New tyres will make a big difference and cost peanuts. Lots of options out there, such as Conti GP4000sII or Schwalbe One, etc. Hopefully you can get 25s on your bike. Then, look at wheels but you will need to factor in your weight and the type of riding you do...as long you are not some big biffa then something like Zonda c17s will make your ride feel zippy but saving the odd 200g won't make you fly up any hill, whatever the marketing BS says.
In terms of comfort, then the above will help a lot more than an expensive seatpost. 25mm tyres and 17mm rims will mean that you should be able to ride lower PSI, particularly if you go tubeless. Weight is a factor there too as that will influence what width/PSI will be optimal but most new riders start with a width/PSI combo that is just plain wrong (say a 75kg rider running 23mm at 120 PSI). Be careful though, most road frames won't take a 28mm tyre...in fact some won't take a 25mm so check all that stuff first particularly if you ride with guards. Don't be afraid to experiment a little, only you can determine the optimal combo and snakebikes are not the worst thing in the world!