I do a lot of work for an independant Lotus garage and what we are finding now is that the S1 is showing its age abit unless you can find a good example. As previously stated make sure you know the history of the car. Check underneath at the front for floor damage, check all the points where the wishbones mount for bent chassis ( gets damaged if had a wheel contact ), as with all cars check the straightness of the body, if you can check all the fixings for the front clam, if they are rounded or damaged it indicates the front clam has been removed for some reason.
If I went for an S1 some of the upgrades I would like to see would be aluminium front rad ( std ones are very weak ), upgrade to S2 dampers or higher spec, had a geo setup.
As previously stated look on SELOC or MLOC as this is where you will find the best examples of cars for sale by people who are fanatical about them
And finally if you get one, make sure u get a new back as after a bit they are a pain in the ar$e to get in and out of.. 😆