im thinking of adding a sub to my boot but not quite sure what to get..
what have other people done?

I am currently looking for a false floor setup. Look into low profile subwoofers or 8's. Search "false floor" to get some ideas.

Here's a pick of an install we did in the later generation turbo'd Cooper S, will work exactly the same in the earlier cars and loses about 4" of depth from the boot. Otherwise if you're really short on boot space, like me, you can build a subwoofer into the front footwell(incomplete, will post pics when it is)

The amp is an Alpine PDX4.150, that was used to drive some front components and rear 6x9s, it's full range digital so doesn't suffer from overheating problems like analogue amps so could easily be accomodated under the false floor. The black discus like object is the In Phase USW10 an underseat self powered subwoofer, others will fit but we are getting awsome results from these and they are a bit shallower than most at 69mm.

I've also seen Phoenix gold's new shallow 10" built into false floors, Pioneer's subs will go in or if you're feeling flush JL Audio do a nice one:)