For what it's worth here's my view as a former owner who sold an ISF (2008) last year.
Firstly let me say that the ISF is an amazing car so no question about it, buy one!
There is a real contradiction where the ISF is concerned. Awesomeness of the car, scarcity of numbers, general excellent condition of the vehicles remaining (mostly enthusiast owned) and amazing reliability. They are just some of the pluses.
But those same pluses mean that owners expect to get a premium price when they sell. Hence why most are over priced in my opinion.
I sold my 2008 immaculate ISF with 40k miles for £18000. I wasn't bombarded with interest but the guy who bought it, like you, had been looking for a while and was blown away by my cars condition.
In summary I believe all owners know how good and rare the better examples are and as such want top dollar. I am seeing the prices stabilise and if anything rising. As such, investing in a decent 2008 to 2010 for anything up to £18k to £19k is a good investment and I don't think you'd lose a lot in depreciation.
Hope my thoughts help and obviously it's only my, previous owner, experience. We all have an opinion and for the record, if I could buy my old one back id do so it in a heartbeat.