Most people's idea of off road riding is nothing more serious than doing a few rough bumpy gravel tracks and having a play in the woods. A basic rigid frame machine from the 1990's is perfectly adequate for this sort of use. You don't need so-called high spec machines just for a bit of fun. I picked up an immaculate 1994 vintage 26" wheel Raleigh for £20 recently - in fact it's way too tidy for bashing around and will end up being kept for "best" while a FOC one pulled out of a rubbish skip will get used as a hack in it's place. There's absolutely no need to spend real money to get a useable bike.
Yes, there are some great bargains to be had with older MTBs, probably because there's been so much development (and changes in fashion) in recent years that there are more people selling off older models than people wanting to buy.
I got a 2006 model GT Avalanche 1.0 a year or so ago for £110, in great condition and with new brake pads (hydraulic brakes). It has basic front suspension, which is more than adequate for my use - "bumpy gravel tracks and having a play in the woods" pretty much sums it up for me.
I also have a rigid Grisley MTB that I got for a mere £30. Its gears wouldn't change, but that was just due to gunked-up shifters which only needed cleaning out and relubing.
When you're going for an MTB at entry-level pricing, you can get a lot of bike for your money by going for a used one. My only real caution is that I'd only buy one I could inspect first, and I'd have to be confident I could fix any problems.