Good luck with the hunt Jase - it's good fun looking
Be careful with a sub-£1k bike as it is just possible that it might cost you more in the longer run.
The obvious engine checks are for smoking and rattles. Does the starter motor sound OK (no graunching noises) and is there a bang on switch off?
I think you want to go green laning so it needs to have:
a sump guard - £50 to £100
appropriate tyres in reasonable condition - else budget another £100 to replace the pair
appropriate gearing e.g. 14/52 or 13/48 and an o or X ring chain - if you have to put on new sprockets then it is a false economy not to change the chain at the same time (unless it is a nearly new O or X ring already) at about £80-100
hand guards - else the first bit of overgrowth will pull on your front brake and throw you off - about £20
Check condition of brake pads and that the calipers are moving freely. A recent MOT should cover linkage, swing arm and stearing head bearings.
If the handlebars are the original steel ones, check they are straight.
On the metal tank models check where the seat rubs on the tank to make sure you haven't got a potential leak problem on the way.
A ripped seat cover can result in a damaged seat foam if left too long.