Even if you buy a really good X1/9 eventually you will find some rust, somewhere.
In the spare wheel compartment you should check behind the sound proofing material as this can disguise holes! That weird stuff could naybe be stuffing from inside the seat??
The clutch might be seized, I have read about various methods of freeing them off. You cant be sure on the gearbox until the clutch is working i guess but the gear syncros are usually weak, parts are available to rebuild the gearbox DIY (£200/$300 in parts+) whilst a rebuilt gearbox in the UK costs approx £700/$1000. Its usually cheapest to try and find an OK second hand box.
I reckon most good x1/9s have iffy syncromesh on at least one gear...
If I was to buy that car, along with dealing with the rust I would fit a new clutch kit, clutch master and slave cylinders. Brake master cylinder and probably rebuild the brakes. This would help improve any chance of reliability
Depending on how long the car has been standing the carb will most probably need a rebuild (presuming its not fuel injection). The auto choke will probably not work, most of them don't as I understand. My car was converted to manual choke when I got it.
The doors are renowned for rusting on the X, although we must remember that most 30yr old cars would be rusted if they were still on the road prior to restoration. At least in the US you have access to dry state cars
If you have a friend or relative who is willing to assist and advise then make the most of it. I have learnt so much from working on my X1/9 and although I'm 29
cry
I have had older cars in the past which were in worse condition.
The typical advice is 'buy the best example you can afford'
I found this great blog last night
http://www.x19blog.com/?author=1 although unfortunately it has not been updated for a while there is loads of really useful info and photos to demostrate the kind of work you would need to undertake with that $400 car.
Any info on this green car?