I had a DCI80.
Nice enough car. If you want something to comfortably get you around, get an 80 imo. Quite a bit faster than the 65.
I wouldn't get the 100 after hearing the problems they can incur with gearboxes, towards the end of my ownership (bought it on 12k, sold on 50k) even my 1st gear was starting to "pop" out of selection if starting quickly.
It's only a 1.5 so lighter than your usual 2.0 diesel hatches, handles quite well with decent tyres.
If you just want to buy one and modify it though, I would probably look into a 65, as they're a LOT more available, which means you'll get a better deal, and can spend the rest on modifying.
As I understand it, a remap and intercooler will take it to 100bhp, which will be quicker than virtually all 1.6s (bar things like Saxo VTS' and 106 GTis) until the top of 5th gear.
This is an 80 with a remap (had 110hp apparently).
0-100mph.
http://uk.youtube.com/watch?v=tyH_BCFZw_E
1500rpm in 3rd to 4000rpm in 4th. (This is the strong point, I did 10 minutes on the Nurburgring with a standard 80, I imagine it would be 9:30 with 110hp)
http://uk.youtube.com/watch?v=WH9sisSJLLQ