Making a VFR C152 IFR

I've just purchased a '79 152 that is currently VFR only. Avionics
consist of a single nav/com and xponder. That's it.

At some point in the next year or two I am going to want to get my
instrument ticket in this aircraft, so I'm already starting to think
about the best way to do that. Trouble is I know precious little about
IFR equipment at the moment. If someone has some good resources/links
that could get me up to speed I'd appreciate it.

Actually, I already started buying stuff. I bought an IFR Garmin 300XL
GPS on ebay (it was a steal). That's my start. I'm completely open to
suggestions on where to go from there.

Maybe a B/K 80 nav? Has everything else I need, right? I know I need
an audio (switch) panel and at least one more CDI.

I'll be searching for an avionics shop soon to install the GPS, but
until then any idea what that will cost me? Might it make some sense to
have them do a VFR-only install initially and then do it IFR when I have
the rest of my equipment? (I want a GPS now, of course, for VFR flight.)

TIA.