This one has been done to death here too, recently.
It depends on how much one is going to use the GPS.
I am going to duck after saying this and get back to doing some real work, but the proper way to use a GPS is for primary navigation. In VMC one continually checks it against the outside view, but the track line on the GPS will be far more accurate and less ambiguous than anything else at one's disposal. In IMC one would use it as primary too, but would also track one or more navaids, if possible.
I have played with PDA solutions pretty extensively (the Mitac 168 yesterday - a great unit for running TomTom in a car) but most are just not suitable for use as one's sole GPS. I have a big panel mounted setup so have looked at PDA solutions as a supplement - especially as one can display the CAA charts on them.
For a start, I have come across just one unit whose display will auto-dim according to ambient light - the Ipaq 5x50, and that works only when on battery, not when powered from an external source. All other PDAs can be dimmed but by fiddling around with the stylus - no good in flight. Most are far too bright for night flight even with the brightness on min.
Their touch screens only need to be touched and the thing goes off in some unwanted mode.
At the budget stated, I would buy a used aviation unit, and get the best one can afford. Also one must consider mounting options - if renting the plane, a leg-strapped unit (not many about) might be preferred to a panel-top one.
Also avoid designs that go back some 10 years e.g. the Garmin 195. They have nasty firmware bugs.
Having said that, an Ipaq 5x50 running something like Memory Map or Oziexplorer, with the screen cover protecting the screen from touching, would be an adequate solution - IF I can find how to enable display auto-brightness when on ext power.