Hi No. 6,
I too have a Birkin, and in fact this is my second! I enjoyed my first Birkin for about a year, then started to look into upgrading (it was a very stock car). As luck would have it, another Birkin came up for sale with nearly all the upgrades I was contemplating, and at less cost than to do the upgrades myself (plus I'm no mechanic).
I've seen a lot of Caterhams and Birkins, and to me either would be fine to own, so it just comes down what car gives you the most of what you are looking for at a price you are willing to pay. Either way, there is no need to rush into a purchase as it seems to be a buyers market with this type of car, and there are always cars coming onto the market.
I looked at the pictures of the Birkin you are considering and noticed a couple things:
This car has the newer Birkins seats which are slightly more comfortable that the older style you test drove. There is no heater installed, which is not problem unless you plan to drive in sub-freezing temperatures. All Sevens generate plenty of heat without a heater! It has the spare wheel carrier but no spare wheel/tire. A lot of owners don't bother with the spare anyway, just carry a can of fix-a-flat. I like the classic look of the spare in the back though. If you like the shiny aluminum look, be prepared to polish it several times a year! It is a nice looking car with the carbon fiber and bare aluminum, and definitly mount the carbon fiber dash to go with the fenders if you buy it.
Good luck with your decision and feel free to ask more questions.
Stan