SPE 1400SX -as rare as rocking horse poop (as you know)
Things to look out for -accident damage, fake ID, rust, hidden rust, disguised rust, & any other type of rust that might be in existence that i don't know of.
Price -that all depends on the condition (i'd guess somewhere between £300 & £30,000!)
Unless you know Minis inside out, upside down, & every which way but loose, your best bet is to take someone along to check out the car with you (& i mean someone who REALLY knows about Minis) as it might not be a genuine SX & apparently roadworthy Minis can be bodged in so many delightful ways.
There's plenty of people on this forum who fill that requirement & some of them live in or near Brighton so it might be worth starting a thread asking if anybody in your vicinity can check the car out with you.
(but don't be surprised if one of them puts in a bid if you're not sure whether to buy it or not).