I have managed to find a couple of people lower end Marins for your price point, Muirwoods and Bear Valley models from the early to mid 90s being the common options.
In my limited experience they seem to be nicely made solid and dependable bikes for the money. Some of the original componentry is a bit agricultural, a few mid life upgrades to the drive train and wheels/tyres can help to lighten things up. Plenty of cheap parts available via this forum.
I managed to get my Muirwoods down the red sections at Swinley without it falling apart, I may not have been as fast as all the downhill boys on motorcycles without engines, but my bike cost well under £100 to put together and it's still great fun.
Thats the spirit! I've always been a fan so slightly biased! Not boutique but well made and "mid range" bikes that come with a decent frame and forks (the heart and soul of the bike) and yes the gear/brake spec is a little under average for the money...but better to have a componants that you can upgrade that a frame....