SteveShannon

One caution about hiring “handymen”, be sure they are licensed, bonded, and insured so that in the event of an accident they or their estate don’t sue you.

No, RF ground and voltage ground are two different things. 
You can still use the mount, but use a 1/2 wave antenna, it will be about 12" in length. I am currently using this antenna on my car https://www.arcantenna.com/products/bmwu4002s

Maybe you don’t, but it does seem to be a popular topic on this forum.

"Full of Smelly Stinky Stuff

At 1200 feet, what kind of wind loads and temperture/weather conditions will the antenna be subjected to?   You might need something more 'commercial quality' to stand up the the conditions at the site.

You’re gonna have to put a budget. Many small repeaters in my area run comet 712efc antennas for $125. And get 20-200miles depending on wattage and line of sight.   However you can spend $1000 on a good commercial antenna sooo…… what’s your approximate budget? 

I usually do both.  I will ask on a forum but also research online.  I like asking because at times people will elaborate what usually works for them and what to stay away from.  Along with that, friendships are often made when asking through a forum.  A tad more human interaction in forums; something that is lost searching videos over the internet.
I hope I did not cause you any distress by asking.

A lot of people on limited budgets or just dipping their toes into amateur radio and/or GMRS will use a hand held radio connected to a magnet mounted antenna in their vehicles.  While a 5 or 8 watt hand held won't reach out as far as a 20 - 50 watt mobile, radio, it still works well with an external antenna.
I ran a magnet mount antenna with my Baofeng GT-5R hooked to an external antenna in my car for a while and had no issues getting into the local repeaters. That allowed me to save up for and research what mobile radio I wanted.

A NanoVNA will work for showing if you are high or low on your SWR and if you need to shorten or lengthen the antenna. And NanoVNA's can be bought for under $100. There is a bit of a learning curve with them but it isn't that bad.

And we’re so glad you grace us with your superiority. 

I always tune for 465 Megs for a decent reading on both 467 and 462 Megs but still take readings on both freqs.
I always tune with a RigExpert 650 Antenna Analyzer and then check the entire antenna system with the radio connected with a calibrated Bird Meter and Elements.

Agree, if you want a good VSWR on GMRS freqs.

It is way easier to shorten an antenna versus lengthening one. This is where a cheap NanaVNA or an antenna analyzer comes in handy. You really need to know if the antenna is resonant above or below where you want it.
I had to trim my Tram 1174 antenna. Before I made a single cut, I measured the SWR with an analyzer to see if it was high (short) or low (long). I made small 1/8" or less cuts testing the SWR after each cut. I did this until I was happy with the SWR.
Remember to test at 467 MHz and at 462 MHz. You might get it spot on at 1.0 - 1.2 at one but be around 1.8 on the other. Or you might get both really close. I got my Tram to 1.3 at 462 MHz and 1.5 at 467 MHz. 
You will be fine as long as you are 1.8 or less on both 467 MHz and 462 MHz. Remember to cut just a tiny bit and test after each cut.
If your lowest SWR is above your target frequency, then the antenna is too short. If the lowest SWR is below the target frequency, then the antenna is long and needs trimmed.

All they really need to do is increase the diameter of the ground plane adapter to the same diameter as the antenna at its base. 

It might not stick magnetically, but it certainly will capacitively couple.  All that’s required for a capacitor is two conductors separated by a dielectric material and aluminum is a very good conductor.
Stick-on steel disks are available to attach magnetic mount antenna mounts to aluminum bodies.

All they really need to do is increase the diameter of the ground plane adapter to the same diameter as the antenna at its base. 

That’s completely wrong. To calculate ERP you take the RF power input to the antenna and apply the gain. 

It might not stick magnetically, but it certainly will capacitively couple.  All that’s required for a capacitor is two conductors separated by a dielectric material and aluminum is a very good conductor.
Stick-on steel disks are available to attach magnetic mount antenna mounts to aluminum bodies.

I got a Nagoya NMO200c antenna I think it's called, for my car. Put it in my mobile, so far it's been impressing me. I've hit the Tampa 575 repeater from my driveway which is slightly out of the repeater's estimated range. It's pre tuned for gmrs (and MURS, never used it other than to test) swr on a mag mount was 1.01 . I'm starting my repeater project now. Thank you for your help, wish me luck!

Trick question.

1. Yes, aluminum will work for a ground plane, like when using a drill through nmo. Many antennas are made with aluminum.

2. No, it will not work with a mag mount. Aluminum is non-ferrous, so a magnet will not stick to it or make capitance with it.

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It might not stick magnetically, but it certainly will capacitively couple.  All that’s required for a capacitor is two conductors separated by a dielectric material and aluminum is a very good conductor.
Stick-on steel disks are available to attach magnetic mount antenna mounts to aluminum bodies.

I wish I knew that. I give away all my radios, but I just gave away this one last week.

The FCC must be wrong 😉

Here’s what the FCC says:

All they really need to do is increase the diameter of the ground plane adapter to the same diameter as the antenna at its base.