Welcome to the forum, Roman. Here's my two cents regarding the Enfield UCE:
1. If you can change the oil , filters and wheel then you'll be able to fix almost any issue with the UCE. My 2011 B5 was the first motorcycle I'd ever owned, and I would not have described myself as a mechanically skilled person when I bought it. But it's been said that the Enfield will make a mechanic out of a man. The statement is less true about the UCE, but with all the helpful advice on this forum, you'll probably be able to maintain your bike as good or better than any shop. As far as reliability, I've had no issues with the engine. My problems have been electrical (battery cables, blown fuses) and related to bolts falling off. All of the problems were easy/moderately easy to fix, and I haven't taken her to a bike shop yet. I've ridden the bike at 70+ mph, taken it for a 1000 mile ride across the country, and been stuck in city traffic with the temp 100F+ without even minor mechanical issues. I've got about 10,000 miles on it and would certainly buy it again. It's the most fun I've ever had on two wheels.
2 . There are several places to get online repair instructions. This forum is one of the best. You can find a UCE workshop manual floating around in pdf format, and Pete Sindal has a CD-ROM manual he put together. I haven't needed anything but the advice I've learned here.
3 . There are several online parts stores for RE. Nfieldgear.com sponsors this site, so I will mention them first. Their customer service can be a little lacking, so I'll also mention Hitchock's Motocycles out of England. I usually buy my parts from them. Ebay has several vendors that sell parts; I've used Bulletwalla in the past and have been happy.
4 . I would say the chain is the UCE's weak link (pardon the pun). A nice o-ring replacement chain is a great first project. Other things I did include upgrading the front sprocket to a 19 tooth, buying a 7" headlight, get a gel battery, and swap some of the other standard bulbs for LEDs. Tire upgrades are nice. Other upgrades that are frequently discussed here include Hagon rear shocks, exhaust upgrades, and new saddles. There are dozens of threads devoted to each upgrade topic, so you can decide which ones you want to do, and then have great info on how to do it yourself.
Be prepared to talk to a lot of people if you buy the Enfield. Almost every time I go out, someone wants to talk to me about the bike. Old Brits, people who's dad's rode Enfields, fans of the retro look, and people who just like different bikes all stop to chat. It's pretty cool.