So based on this do you think 50/19 is too highly geared?
Only you can say ;-)
Do it for a few more days and see how you get on on different days and weather etc. And don't over think it, even if you do get your gearing 'wrong' you can change it later for the cost of a chainring or freewheel so not the end of the world.
As for spinning, you WILL get better at this if you go SS/fixed, you'll also realise that for a lot of downhills you'll get more benefit out of a good tuck and coast than frantic spinning.
The cheapest way to do it is normally a home build out of spare bits and an old frame, or accumulated/borrowed parts from friends. But this can result in a bit of a bodge job or actually end up more work than you expect if you're not sued to all the idiosyncrasies of what will fit and what won't so sometimes it's better to either bite the bullet and buy something new and 'done for you' or at least look at 2nd hand options.
Be aware that a lot of SS and fixies are more track-esque than road-esque, so have very racey geometry and lack provisions for guards and big tyres, so be careful if you spot a bargain that it's not actually totally inappropriate!
I've had Langsters in the past, they are nippy, tough and ride well, and available very cheap 2nd hand, but you'll only get a 25mm (maybe 28) tyre in there and they are a pig to get guards on.
The Genesis Flyer is a decent all rounder to look out for, takes 25/28mm with guards and is sensibly built, but not often easy to find second hand.
My current SS is a 1980's Mercian* 531ST frame, with some home-built wheels, and it cost me under £200 all in, but I did already have the wheels... but decent old frames are easy to come by and as long as you pay attention to things like making sure it has horizontal dropouts, the right clearances and braze-ons etc. you can put together an excellent bike for very little cash.
*less well known builders or mainstream brands can be found even cheaper and at equal quality.