It would not be worth converting one Mark. The pack is quite different, with the exhaust exiting on top of the hull rather than to the side. The instument and control panels are also quite different, as is the fuel system.
I wouldn't write off a Mk 1, most were used as hard targets and the ones which weren't have mostly ended up in the USA, where petrol is cheap. There are not many good ones left in the UK. A friend of mine has his mint Mk 1 on milweb at the moment, it's the deep bronze green one we took to Beltring in 2006.
If you want to stick with a Mk 2 then I'd go for the Mk 2/1 with the internal NBC pack as it takes up a bit less external space. We had both a Mk 2 and a Mk 2/1, both sounded great and ran well, but I find them a bit large and slow. They will do a steady 32mph, which isn't a bad turn of speed though. I also don't like not having a seperate brake, having to stop with the tillers is fine, but if they aren't set correctly or you pull one slightly more than the other you could end up in a messy situation!
They are nowhere near as manouverable, fast and manageable as a CVRT. You will need some heavy lifting equipment to get the pack out, and a lot of strong friends should you need to do track bashing. Remember that it's not just pads to worry about, but you need to check that the pins and bushes are also in good order. Also watch out for rear idler hub failures, the guy who has our 432s has had a few hubs go and it's a big and painful job to swap one.
Hopefully I haven't put you off, if you are still interested can I suggest you look at a FV434 instead? There's a decently priced one on milweb, now that is a much more useful toy!
Chris