My mum had a similar saying, "buy cheap, you buy twice"
Welcome to the crew, you'll love the trails at Redhill, there's plenty there and it'll get you fit with all the climbing.
Definitely wait to see what you really want, it takes time researching, riding, and demo'ing to get it "right", and even then it's a compromise in most cases. Even with the uber bikes you see people buying.
And when you do get something, keep it maintained. One of the most frustrating aspects is wheeling the bike out for some action only to find something is playing up.
Finally, body protection - get some. It doesn't always help but it can save the day when it does. Knees and elbows in particular (and shoulders).
Good luck and have fun.