i would recommend the Samsung Omnia from expansys.com on TMobile network which includes 1Gb of data allowance per month.35 a month. this phone also has wifi......
I haver an MDA PDA from T-Mobile. It has windows 6, CoPilot satnav built in, and hsdpa for internet access. It is also a touch phone and has a keypad and on screen keyboards in various styles to use with the stylus.
I pay £35 a month for web n walk. It says unlimited internet access (reaonable usage 3GB) but I have had it hooked up to my laptop all or most of the day, every day for a month and not been kicked off! You can also access the internet (and your emails) via the mobile itself as the screen is quite big. The only drawback to the hsdpa connection is the graphics are not perfect on the laptop, though they are reasonably good, but you couldnt use it to check design work properly.