@Yuri1976
Phew! That's a lot! :-)
(a) Have a quick look at BulgarianProperties (dot com) as they have a lot of listings all over Bulgaria, and they specialize in expats. Ebay also good for cheaper village houses (and there are some "pay-monthly" options which are very popular).
(b) The easiest is ready to move, but that's the most expensive. You can buy a village property which has a nice garden and has water/electricity already connected. You can renovate the house, or you can build a new house to the SAME FOOTPRINT without any bureaucracy. A regulated land plot can be an option if you want to build, but it's a lot more work. Prefabricated houses are pretty good now, and some of these companies will take away a lot of the headache (as they often deal with the planning permission, and do most of the construction work).
(c) Tax here is a flat rate of 10% for both personal income tax, and corporate income tax. You can be self-employed or have your own EOOD, and I'd expect most accountants to recommend the latter. It's not as online and easy/cheap as the UK, but you can quickly and easily incorporate a company through an attorney.
(d) After the fall of communism, English is now taught in schools. So younger folks tend to speak English. Google translate is pretty great. And you can take a freelance (state-registered) translator with you for around 50 leva per hour. Like everywhere, there is bureaucracy, and it's a pain... but you can navigate on your own. But the low-stress option is to take a translator with you. Or, many of these processes (e.g. residence permits, re-registering vehicle) can be delegate entirely (for a fee) to an attorney.
(e) Healthcare seems pretty good here, but I don't think I would go to a public hospital (it's a lot more crowded, and usually not quite as modern). There are some great private hospitals and I'd guess the care is good. If you buy a private health insurance policy, you're covered at private hospitals. If you're covered in the public health system, you can still go to most private hospitals and they charge the "upgrade" fee (relative to what the state pays them). I registered in the public system as unemployed, it costs me 13 euros per month. So I can register with a GP, and I'm entitled to free (or nearly free) treatment at a public hospital... or I can go private and pay a bit extra. They also gave me an EHIC for use elsewhere in EU. If you register as I've done, then you can still buy a private health insurance policy, but it will cost a lot less.
(f) It's a great country, a bit of an undiscovered gem. I don't think there are any nasty surprises waiting for you. Friendly people (especially in the villages), good food, and spectacular scenery. It's not cheap for Bulgarians, but if you're an expat with money (or making good money remotely) then you can live very well. The official minimum salary here is about 360 euros per month. A full UK state pension will be around 1,000 euros per month, so while you can't live like a king, it's not bad (especially if you sell a UK property and buy here for cash, so no rent/mortgage to worry about).