Is this the antenna you are looking at?
Commercial, heavy duty vehicular antenna, Multi Band Mobile VHF/UHF/700/800, 136-174/380-520/760-870 MHz, 100 Watts, Whip Length 20"
theantennafarm.com
If so, those are overpriced and butt ugly. The base is rather large diameter, almost to the point of being comical. They were one of the first multiband antennas that hit the market for the new multiband transceivers.
There are better and less expensive options:
I've been running some EM Wave antennas for a while now, but not this specific one. EM Wave makes nice stuff and I suspect this one would be a good performer:
Tri-Band 150-162/450-490/763-870 MHz IP67 Rated Roof Mount Antenna
theantennafarm.com
I've been running this antenna on my work truck on both VHF and 800MHz, and it works well. The base is much smaller and that puts the spring closer to the vehicle roof. I've run up some gnarly site access roads and it's twanged a heck of a lot of tree branches with no issues. I also did a review on these a while back where I swept them on an antenna analyzer. It's been a really good antenna:
Larsen NMO150/450/758 Tri-band Mobile Antenna covers 150-174, 430-520 & 750-870, 100 Watts, Length 16.5 in, NMO Mount
theantennafarm.com
There's a lot of scanner listeners running this antenna, and it gets consistently good reviews. It doesn't have a spring at the base, however.
Tri-band antenna covers 150-165, 450-470 & 806-940, 100 Watts, Whip Length 16.5"
theantennafarm.com
There's no way I'd spend $110 bucks for that Laird monstrosity. There's better antennas on the market that will last a lot longer and do less damage to your vehicle if you hit a low branch.
As for mounting it, if you are willing to spend that much on an antenna, I suspect you are taking the performance of your radio seriously. If that is the case, then the right way to do this is a permanent NMO mount dead center in the roof of your vehicle. That'll work for any of the above antenna (even that God-awful Laird). That will give any of the antennas an ideal ground plane under the antenna and give you the best performance you can get.
Popping a hole in the roof of your vehicle is a big decision, I get it. Not everyone is up to it. But once you do your first, you'll realize why so many of us suggest it. It's a 'no corners cut' setup, and it'll give you the best performance. Any other installation will be a compromise.
If you don't have the tools, knowledge, or skills to do it yourself, there are likely local radio shops around you that would do the install for you. It really is worth it.
If permanent install just isn't an option (I get it…) then give us an idea of what your budget is, what you skill level is, and what kind of vehicle you drive and we can give you some suggestions.