HTC Touch Pro ... its basically an N900 (has most features) by HTC released few years back.
You can grab a used one for around $200, then install custom ROMs on it.
From what I know, xda-devs have already perfected the Android ROMs for you, but there are WinMo6x ROMs available too if your into it.

The noticeable differences are:
- a smaller screen with less pixels (640x480)
- a slower processor
- graphics chip without 3D driver (unless you run a custom ROM from xda-devs which have this fixed for some PSX emulator fun)

Then it has some differences which matter less:
- a decent camera but not as good as the N900
- no internal memory which doesn't matter since it has SD support
- no infrared (which doesn't really matter)
- no FM transmitter (which doesn't really matter)

The similarities are:
- form factor (ie slide QWERTY keyboard)
- a front camera for video conference
- LED flash for back camera
- resistive touchscreen
- 3G and QuadGSM
- aGPS on both
- TV out
- Gsensor/accelorometre
- RAM (288MB HTC vs 256MB N900 ... 756MB virtual RAM does not add to performance)


So when it comes down to it, the Touch Pro is an underpaid device.
Go grab a bargain!

PS: I don't want to get your hopes up but this device is powerful enough for Mer/Maemo/MeeGo but there is no attention to it.

Your other option would be the Motorola Droid which is better than the N900 in screen (size and resolution), capacitative touch and slightly more compact ... but it isn't in your budget!