Welcome to the forum.
The frames were not protested from rust when new so that can be a problem. The frame and rear suspension trailing arms are only 16 ga. sheet metal, so anything beyond minor surface rust will need attention. Cracking around the stress points of the 'T' section at the front of the frame can also be an issue.
It sounds like you may not be able to get under the car to check this stuff out, though. If you are handy with a welder then minor repairs can be made. If the frame is shot, there are several choices on the market for replacements, so not necessarily a show stopper.
As mentioned, these cars are quite simple and for the most part quite easy to work on. Parts availability is great and the cost is not ridiculous in most cases. A lot of Triumph, MG, Mini, etc parts make up the period Lotus. The parts cast from unobtainium are the gear box and the rear suspension uprights/bearing housings. Non OEM solutions exist for the gearbox, though. There are some parts that are difficult but not impossible to find (mostly trim items, etc), and the vast majority of consumables are as close as your nearest classic British parts supplier. If you think a Lotus should be in your future, it is quite doable.
Looking forward to seeing some photos and hearing more about the car. Good luck.