Both Espar and Webasto are comparable units. If you need to deal with altitudes, then the Espar has a $300+ high altitude module that will automatically compensate for altitude up to around 7000 feet. Supposedly the Webasto are good to about 5000 feet before having an issue, but if used regularly in higher altitudes should be adjusted. There is a hack to do so, and that is the route I went with my diesel powered Webasto. Gas versions from either brand will be more prone to the fouling issues related to altitude than the diesel ones, or at least I am reading that quite a bit in various places.
Another thing to consider, which will be less of an issue with a gas version, is the electrical load to run one. You have a starting sequence which has more energy draw up front (especially on the diesels), then a fan and pump to run after that. If you don't have a backup battery, additional capacity, etc. you will want to account for that so that you are not left stranded somewhere by killing the starter battery.
Planar and Snugger are two lower priced e-bay units that people have been having success with, but long term reliability, and long term exposure to altitude (if that is even a concern for you) are not fully known. Plenty of anecdotal info out there such as Victorian's post above about having no issues at 8500', so your you may want to take the chance. How they hold up at altitude over time is anyone's guess.
My spend for a Webasto diesel unit with the low end rheostat temperature dial (needed for the altitude hack) was $1000 vs. a similar setup from Espar that would have been about $1400 with the altitude add on kit. It included everything needed for the install such as controllers, wiring harnesses, fuel pump/line/attachments/anchors, intake and exhaust hoses, fuel tank tap, etc.
Any one of the lower end units should have more than enough heating power for the situation you describe. Good luck.