oh, never played with an hp tablet
what your talking about with taking it apart & extending the wires & building everything in to the dash is very doable, but if you don't have some experience & pretty good fabrication skills & the resources this can be a big undertaking. do you have any experience with that kind of stuff? a good example of this kind of install is in my signature:"modded infiniti fx with big screen"
if you take apart a tablet & embed it in your dash, I think that makes a great carpc, but if your looking for easy I don't know if I'd really suggest that route. extending many modern ribbon cables can be a huge pain & sometimes is not even possible without specialized equipment & custom plugs, this can be a big problem. I usually try to work within the stock cables reach, & have always found a way to make that work. again, a good example of that is in my install. I've done this over 5 times with different cars, extending the cables is a last resort with some cable designs. the gateways & dells are easy to extend & the Fujitsu's are almost impossible. the older toughbooks were easy to extend...
sometimes you really don't know what your up against till you buy it & crack it open. this stuff is very delicate too BTW.
the biggest thing I'd have against the tablet you choose is the screen. definitely look for a tablet with an indoor/outdoor screen. motion, Fujitsu, panasonics & a few others all make them. this should be the #1 criteria you should use in choosing a tablet. a slow tablet with a great screen will make a better carpc than a fast feature packed one that you won't be able to use during the day.
your budget is pretty low... & not a lot of spare time don't sound too good either... if your not going to be able to dedicate the time & energy & resources needed to pull something like this off I'd say really reconsider what it is you really want to do. sticking a 7" screen & a used cheaper laptop shoved somewhere may make more sense, & modular means that latter when your budget improves you can upgrade it piece by piece