What's more important than the right tool is the training to know how to use the tool. Within reason, any of the combustion analyzers will do the job you need to accomplish if you know how to run them.
Check into some training at http://www.nationalcomfortinstitute....ontraining.cfm before you buy a gadget to do it. They offer a bit of a break on their equipment after you've done the training, and their stuff is some of the best.
Till then, keep posting, get your pro status and you'll be able to do a lot of reading on combustion tuning.
"If you call that hard work, a koalas life would look heroic."