Providing the condition is right each of those bikes will be fine to get into mountain biking, and around £100 is a reasonable price used. I had a GT Aggressor for a while and it was a nice bike. As you say, the Trek 810 is the oldest, but that shouldn't be a problem if the condition is good.
Just make sure that the bike fits you and that basics such as gears, brakes and suspension work OK, and of course no serious frame damage other than possible scratches or minor dents which you often get with used mtbs. I believe all these bikes are alloy so rust shouldn't be a problem.
You will learn about maintenance as you go along. There are plenty of internet sites that give instructions and/or videos on most types of maintenance if you search for them.