Do -NOT- use a hacked/modified amateur radio for GMRS.
1. It's illegal, the amateur radios do not have type certification for any services. Amateur doesn't require type certification for transmitters, even mass produced ones. They do have Part 15 type certifications, but thats for the receiver only.
2. A lot of amateur gear is 25KHz FM channel width only, and there are situations where you may need narrow band FM, especially if you need to talk to the consumer grade FRS and FRS/GMRS radios.
3. Some amateur gear has filtering on the transmit side that may not play well with transmitting outside the amateur bands.
4. You can do better, and you can do better legally.
Part 95 compliant radios are out there and are plentiful. Most of them will cover the 70cm amateur radio band just fine (and legally!). Many of us who have/had GMRS and Amateur licenses have done this. Most modern commercial radios have a lot of memories, so it's not difficult to program in 70cm amateur radio repeaters and simplex frequencies.
Commercial radios come with handy features that amateur gear doesn't have, like off hook PL decode. I found this -very- useful when I had access to a busy repeater. The repeater owner assigned me my own CTCSS tone, so I could set up RX CTCSS on the radios so I'd only hear traffic from my family. Taking the mic off hook switched it to carrier squelch so I could hear if the repeater was in use before transmitting. Also, while annoying in some cases, options like MDC1200 or FleetSync can be handy for radio ID's.
"Best" radio depends on a couple of things that you haven't provided information on.
1. "Best" depends on your budget. The "best" $250 radio is a lot different from the "best" $1000 radio.
2. What is your installation area like for your mobile? Do you need a remote head?
3. What's your experience level? Have you programmed commercial radios before, or do you need the radio programmed for you?
4. What's your expectations for coverage? Be realistic here....
There are a lot of really excellent radios out there that are perfectly legal and will do what you want. Having realistic expectations and an appropriate budget means a lot to picking one out.
When developing your list of expectations and your budget don't forget the antenna system. For mobiles, you really do need a decent mobile antenna, preferably permanent mount NMO. If you are planning on using a radio as a base, you need to consider a suitable base antenna, feed line, lightning suppression, grounding, antenna mounting, power supply, etc.
As for receiving, adding a scanner to your equipment will open up your options quite a bit and may actually get you a better overall system rather than trying to fit all these functions int one radio. It may also be a lot cheaper if you want to listen to services that are running P25, DMR, NXDN, Trunking, etc. This also lets you keep your two way radio available, rather than tying it up listening to other traffic.