I think that there is other gear that is more important than a HiWatt amp to you get closer to that magical David Gilmour tone. In particular: 1) strong strat pickups and 2) a Leslie-type effect.
1) Gilmour now uses Kinman Woodstock Plus strat pickups. These are noiseless and have a great tone. They are also more powerful than normal strat pickups - nearly as strong as Gibson P90s, which means they are very well suited for BJF pedals (which I've heard were designed to work best with Bjorn's P90 equipped flying V). I have a set as does my brother (nichcope on this forum) and we love them.
2) To get that great swirling sound from songs like "US and Them" a real Leslie would certainly do the trick, but these are super heavy and expensive so probably not practical. I have a H&K Rotosphere which is the next best thing (awesome sound), but it is expensive, big, probably fragile (it uses a tube), and it needs a special power supply. It also has only 2 speeds: fast and slow. Despite these shortcomings it does what it does so well I still recommend it. More practical alternatives would be a BJFE Mighty Green Mini Vibe or the Mad Professor Tiny Orange Phaser. These won't be able to exactly replicate Gilmor's sound, but they can come pretty close and they also do a lot of other Hendrix, Trower, SRV and unique tones that you can't get with a Leslie or H&K. Here are a few links so you can hear some examples of what I'm talking about:
David playing "Us and Them" through what looks like a Leslie - note he's using a Fender Bassman not a Hiwatt here:
http://www.youtube.com/watch?v=NqUX4oPy ... re=relatedFloyd tunes with H&K instead of a real leslie.
http://www.youtube.com/watch?v=D1A-B72_goEBJF and MP pedals on this one (esp tone he gets at about 5:15):
http://www.youtube.com/watch?v=1_BWEBPneJ8If you still want a Hiwatt amp, I very nice '82 Hiwatt 50W head (a great amp) that I would sell for $1100 if you are interested (I'll list it pretty soon on TGP along with several other old amps that I need to sell to pay for my new Louder&More and Mad Professor CS40 which are arriving soon).
I hope this helps.