Maintanence for the PPG boxes isnt to bad, from memory dog rings were around $300 ish and dog gears $600ish which is pretty much what you are going to wear out, shafts etc should last forever unless they get damaged.. No idea what sort of kms you would get from them in a street car but if you were nice to it (double clutching & rev matching etc) you would probbaly get quite a few kms out of it. I'd like to think they are quite streetable, the whine from the full straight cut box gets anoying (yes it sounds awesome, but it gets old pretty quick) I belive they do a semi straight cut, which would be a better option for a street car. It shifts similar to a stock box, just with added clunks and bangs..
other things to think about, you will need a new clutch as the spline on the input shaft is based on an evo 6 input shaft (10mm larger than Evo 1-3/VR4), this may mean that the gearbox case needs milling out to accept the larger shaft. Also there is some variations in the detent spacing on the selector rails between each version of the gearbox, get it wrong and the dogs wont engage propperly and it will go bang.... I'd suggest spending the money to have PPG's expert box builder Brian build it for you (its around $900 but they charge a flat fee to assemble) ..
Albins make a 1st-5th helical cut dogbox for VR4 - Evo 3, its about $5500, however it includes a lot of non standard parts, like bearings and seals etc, that only albins can supply. I know there stuff used to have a bit of an average track record with the rally guys, and i have heard a few bad stories however i do belive they have improved some what. Personally id think the PPG box is worth the extra cash.