I am kind of interested in this as well, but have a limit of about £100. What I have been looking at is
HTC Desire HD
Samsung Nexus S
All three have decent developer communities. The Atrix runs a dual core Tegra 2 with 1gb of RAM. They seem to be very cheap even with the "laptop" accessories.
Looking at the £150 limit, a HTC Sensation could well be about that price. However you may be lucky and pick up my phone (well not mine as I want more than £150), a Galaxy Nexus for about that kind of money.
It's a hard one. The Motorola Atrix is 4G and very cheap and can be rooted with ease. The mind boggles.