Just last week I installed an aftermarket stereo that I purchased from Crutchfeld. I have no affiliation with the people at Crutchfeld, but I was very pleased with the product that I purchased and the techniical phone support that they provided.
You can do it yourself and do at least as good a job as a professional shop at a fraction of the cost. Probably better if you take your time.
The installation is not just a swap in as the old radio (my 1985 had a Hitachi unit factory installed) is considerably larger than the DIN unit that you will use to replace it. You will need an adapter plate that replaces the face plate of the factory unit. The antenna height adjuster switch mounts to the replacement adapter plate. You will also need a wiring harness and an interface module. My 1985 model had a speaker grounding scheme that required the interface to get everything functioning properly.
The folks at Crutchfeld had everything that I needed and shipped in two days. I made two phone calls to get clarification on a couple of minor things and was done.
The biggest challenge I had was getting the new wiring harness and interface into the space left in the dash.
Good luck, be patient and READ, READ, READ the installation instructions thoroughly!