Nice find, I love these so much I'm itching to get another but I tell myself NO.
You're going to have trouble with buckets and MX5s. The subframes do NOT allow for much room at all. I tried buying 2 sets of different bucket seats on ebay and neither of them fit.
I forget the name of them, but they're the buckets that you put in Elises, they're very thin (or indeed just regular Elise seats fit). But you're very restricted in choice, doubly so with a cage fitted.
As far as boosting goes, you won't be getting 190bhp with £1000, not unless you do some stealing or buy 2nd hand from someone who doesn't know what it is. You can pick up the JRSC for a 1.6 for around that price, but I believe it's hot side so no intercooler and that gives about 150-160 horse.
Capable supercharger kit for the 1.8 is about 2.5k new (although this may be out of date as I haven't looked for a while), but sounds like this: