I did my instrument training a plane with a Garmin 430.   That is the way to go.  I would love to put one in my 172 but it's too much money right now. You don't need the ADF because most NDB approaches have a GPS overlay and you don't need DME because the Garmin can be used as a substitute on most approaches.  You only have to do three approaches on the checkride and you can do ILS/LOC/GPS.

Keep your handheld GPS for situational awareness in case of electrical failure and pick up a handheld radio for the same reason if you are going to fly any heavy IFR. Good luck.

I don't know if you are planning on getting new state of the art kinda stuff or would consider previoulsy owned stuff.
One unit you might give consideration to would be a KNS-80.
It's a fully integrated RNAV system with internal VOR NAV, DME, LOC, and Glideslope systems
<http://www.avionix.com/rnav.html> for more info..
Just a thought...
Michael

Paul,

Just to be cute.  The regs say (and I paraphase) you have to have the appropriate navigational system to fly the approach, e.g. to do an NDB approach you have to have an operable ADF on board.  BUT the regs never said you have to turn it on!

(You'd be foolish not too and it isn't he spirit of the rule; but I thought I would share that with you!)   

I tried this on a flight but I turned on and used the ADF to fly the apporach.  I found out that my handheld GPS (Garmin 295) and ADF disagreed considerable when finding the IAF (the NDB).  The Garmin approach starts the NDB approach about 5 nm from the airport on the correct bearing, it doesn't use the NDB or have a IAF. It uses an artifical FAF. Considering you are above and within the MSA it would probably be safe, but not a good idea.  What does the Garmin 430 (panel mounted IFR certified GPS) do with an NDB approach)?  Does it even have one?