Enroute GPS Install Cost?

Can an approach-certified GPS unit be installed but certified only for
enroute IFR use?

WILL a shop install an approach-certified GPS unit to enroute IFR
certification only?

Is an annunciator panel required for enroute use? Is an OBS resolver
required for enroute use?

I'm trying to minimize the cost of bringing my plane up to minimal IFR
capability. Right now, I have a Garmin 250XL GPS/Comm (VFR only), and that's
it. I'm thinking of adding a Narco NAV122 so that I can shoot my 1
precision and 2 non-precision approaches (ILS / VOR / LOC), but I really
like my 250XL.

I don't think I can use my VFR GPS for flying direct enroute - or can I?

If I can't, I was thinking about swapping my 250XL for a 300XL, but I really
don't want to spend $3,500 to install and certify it for approach use. Since
the connectors are the same, making the physical install a simple slide-in
transition, how much more would be required to do a legal install for
enroute certification only?

Alternatively, how much to install an enroute-only GPS, like a UPS/Garmin
GX55?

Thanks;
Carl