IR guns are not completely necessary but they are definitely recommended and are extremely useful tools. While your probes will provide you with general ambient temps an IR gun will allow you to measure specific areas on the fly, like, for instance, the temperature on a branch 6" above the enclosure floor. As for cost, you can find them well under $20, although I'd be more inclined to go with one in the $20-30 range based on reviews. Still, that's a small drop in the bucket and a well worthwhile investment, IMO. Run a quick search on amazon OR check out harborfreight.com.