You could get Win7 for a 64-bit system and update it to 8+ eventually-that may possibly be the cheapest way to do it. The older the operating system is, the less support it will have in time, but I think 7 has a few years left. 8 boots up quicker, but aside from that, it is not very different.
8.1 is just windows 7 underneath, optimised with touchscreen support and a start screen, and aimed at the home market. There are arguments in that 8 is faster for gaming, but tests from differing websites show that it can vary. There is no straight answer.
The main issues some people have with 8.1 with games seem to be related to driver support for their hardware, but compatibility with 8 and 8.1 is still very good with games that run on 7. If you don't want to use 8 apps shown on the blue metro screen (think of them as 'full screen gadgets') then you don't have to. You can stick to using the desktop and you are not missing out. If you want a start menu in 8, you can install classic shell or start8.
Modern games from now on should be set to work with 8.1. Most simple indie games should run fine if they worked on 7 and 8, and if any popular mmo's have problems, they will be updated to work on 8.1 and beyond so that they dont lose custom. Every pc nowadays is being sold with 8. Much software will eventually make the switch to being for 64-bit only, such as photoshop.
If you are not sure if an old game will run, check their official website forum or google 'does X work on 8.1' to find out if someone is running it ok. There will always be people with problems when you google that kind of question, but it will at least give an idea of what the worst possibility could be. For old FPS games, online shopping is your best bet - many old FPS games will be set to run on 8.1. gog and steam have FPS games such as far cry 2, medal of honor etc that have been tweaked to work fine on 7,8 and 8.1