Just to chuck in my 2p with regards to VX versus S1. The VX is more akin to the S2 with a bit more refinement inside. Also best trying to get into both with the the roof on, or perhaps more importantly let your girlfriend/wife (sorry can't remember your staus) have ago at getting in. Like an S2, the sill on a VX is a bit lower and more user friendly.
I was in similar postion to you recently and was thinking either a cheap S1 £7k-£8.5K for thrashing or a cheap S2 at about £12.5k. I really like the S2 and must be getting older as the refinement appeals to me. However, I did have a hankering over a 135 (again) or a 111S just for that we bit more power and close ratio box. I realised that these models in S2 would be well outside my budget. It was therefore just chance that Lee decided to sell his S1 135 Sport when I was looking. As it was my old car I knew the history and was obviouly a good buy at the money.
It is a buyer's market at the moment. You may have to travel but I think you are right, eBay is a pretty fair barrometer for gauging the true worth of cars. Owners on the list may disagree with that but I have been watching the same cars on pistonheads for weeks not moving with the adds simply getting "refreshed" to put them back at the top of the list. You never quite know how much the sellers have had to discount the ones showing as sold.
If it is a S1 135 Sport you are after make sure it is a legit car you are getting. There were less than 100 of these made but there were also a lot of standard S1's which had factory upgrades to the engines. I am sure these will also be good cars, but just to make you aware that they might not be quite what you are expecting.
Finally one thing that swung it for me a bit was a thread that I read recently on the BBS (I think) that talked about the S1 being a true classis and perhaps holding value better. It was well written and sounded plausable. Then again I probably liked the sound of it because that was what I was needing to hear at the time
Good luck
Stewart