Panel mount IFR GPS for training ...

Last weekend I bought an IFR equiped '68 Cherokee 140 to get my
Instrument ticket in ... ( I flew it from Bedford, MA home to
Savannah, GA in one day. But, that is another story. )

The guy that I'm going to use as my CFII came out and looked the plane
over and told me that while the plane is technically IFR equiped it
still needs a few things. ( Isn't that typical??)

The plane is equiped with a KMA-24 audio panel with markers, KX-155
w/KI-209 nav/comm with glide slope, KX-125 nav/comm, and a KT-76
transponder. I also had pitot heat installed during the prepurchase
annual/inspection.

According to my instructor I can get my instrument ticket in the plane
as equiped, however in order to be a more well rounded instrument pilot
I really need to add an IFR certified GPS to my panel.

Question #1. In order to get my Instrument ticket tucked safely in my
pocket do I really need to be concerned about learning GPS approches,
etc.?

Question #2. If I do decide to add a GPS to my panel what is the
best/least expensive way to do it. GNS 155XL? KLN-94?

I don't plan on keeping this plane forever. And I don't want to invest
a ton of $$$ in it.

Any comments, suggestions, experiences would be appreciated.