The workman HP201s or if you can find one, a radio shack SWR/watt meter. The Workman HP201S is about $38 on Amazon and the radio shack meter if you can find one in a store are $40. You can also find the radio shack ones on eBay as well. I use the radio shack meter myself when needed and it works well for both mobile and base use due to its size. The Workman meter is a larger meter. Some say they aren't good, some rave about them. And as far as antennas go, stay away, far away from the k30 or k40 antenna! They suck! The one I took a pic of was just for reference. The Sirio performer 5000 or the Sirio 5000 trucker series are both good antennas. Many members here use them and will vouch for them being a good antenna. They are about 7ft tall which is not shabby at all. And like I said before they are well built. But like I said, stay away from the K30 or even the K40. Length is key to a good antenna as well as the build quality and the Sirio antennas are good in both areas. No it's not a 102" whip, but it's darn close to one. Do a search on them on the forum here and you'll see that there are quite a few members that use them. I would recommend a hard mount over a magnet mount, but if you are dead set on a magnet mount get the Sirio PL145 one. It's made for the Sirio antenna. That or get a tri or quad magnet mount. A good hard mount that I have recommended to many is the Breedlove 3" one hole SO239 mount. They are built very well and work awesome with the Sirio antennas. Drilling a 5/8" hole is what is required for one of them. And if you decide to sell the vehicle, buy a cheap Wilson gum drop mount and replace it with the Breedlove one. Or even one of the shark fin type radio antennas would work as well. Don't be afraid to drill your vehicle, as in the long run the hard mount will be the better way to go. Less maintenance. With a magnet mount you constantly will have to constantly clean under the magnet and also make sure that no water or condensation or water gets under it either. Just some food for thought. Not saying they don't work. Just more work involved with keeping them working right. A hard mount is a one time deal and forget about it. And the Breedlove mounts are top notch. They cost more than the magnet mount, but I promise they are worth every penny!!
Hope all this info helps. Let me know if you have any other questions. And oh yes, as far as coax goes. Make sure you get some quality stuff. And I don't mean the truck stop coax!!
Belden, times microwave, andrew, commscope, shireen, Judd wire, 123 wireless, Davis RF, and a couple others make quality coax in all different types. Me personally for a mobile, I like LMR240UF. Yes some will say it's over kill, but it's good coax! These are all JMHO's, and some will argue that I am wrong. But most will agree that I am telling you the right stuff. Yes there are more expensive watt meters out there for sure. I just named off a couple in the price range you asked about that seem to work well for what they are. They are better than using the internal SWR meter no doubt!! And I am sure about that 110%!! That said, again, hopefully I have given you some decent info to get you going in the right direction. Have a good day.
73 and God Bless
Sean.