I used to love the d-pad on my 6800 (mogul), and then found the TP1's unreliable. I switched to a Treo Pro too, but flip-flopped between the TP1 and the Troe for some time...until the TP2 came out.
I had the exact same fears and concerns you are having now e.g. lack of front-facing keyboard, lack of d-pad, etc. I soon got over the keyboard fear. The slider keyboard on the TP2 is far superior, IMO, to anything else out there in the WM arena. Although I liked having a front-facing kbd on the Treo, I actually found it cramped and prone to typoes. And once you see / use the screen on the TP2 you'll never go back to the 320x320 SQVGA on the Treo
As far as the d-pad is concerned I did miss it at first, and for the same primary reasons you seem to be mentioning...flipping throuh e-mails. However, you have two ways to go. First is as Cicatrize mentioned, and that's the finger slide motion. It is not too bad at all once you get used to it...or more accustomed to using your finger. Second is as another poster mentioned above and using a keyboard mapper. I use the Rhodium Keyboard Controller over at xda and
have remapped my volume up / down to actual up / down actions, except in the phone app where they still function as volume buttons. I also remapped the BACK key on a long-press to the Enter action so I could select / open e-mails once I'd moved down to them using the up/down buttons. Works really well IMO and is a fantastic must-have app for these reasons / uses and much more.
Good luck!