I would contact avionics shops for a quote as the cost of the install
will be substantial and they may be able to get you a discount on the
equipment if they do the install. I just had a audio panel (with
intercom) and a second nav/com and indicator installed along with some
old radios removed, an avionics master and ptt's installed for $1800
with me supplying all the equipment (+$3K used). You'd be looking at
quite a bit more in labor for an IFR approved GPS installation plus the
cost of the equipment. The quotes for the install were all over the
place so shop around.