Mileage is not the primary determining factor... yes, it can help you pin-point pontential problem areas such as worn clutches, idler wheel bearings nearing the end of their life, etc..... BUT, how the sled is treated and cared for means way much more than mileage. I have an Ultra SKS with over 3000 miles that looks like it came off of the showroom floor last year. (I'm the 3rd owner) However, I have a buddy with a same year MXz 670 with only 600 miles that looks like it has been through ten owners and 10,000 miles. (He bought it new btw.) So, if the sled truely has 1800 miles, it has been some hard miles, with little care to the machine. I would look deeper, into things like clutches, bearings, chaincase maintenance, etc... things that could potentially leave you stranded. I've seen tracks with thousands of miles on them, with minimal wear. In the same token, I've seen tracks with only a few hundred miles with multiple torn lugs from spinning the track on rocks, stumps, logs, etc.... Mileage doesn't tell the whole story. Broken sled speedo cables are common, so sometimes you take the mileage on the speedo with a grain of salt, cause the condition of the sled will or may tell you the true story.
Any used sled is only as good as the previous owners have treated/ridden it, and maintained it. A well beat/worn sled with 1000 miles is gonna be worth less, in my opinion, than a sled that is impeccably maintained and well cared for with 4000 miles.
Anytime you are buying used- Inspect the sled carefully, look for potential problems, ask around if you are unsure- and get an educated opinion. Research what similar sleds are selling for, then pay accordingly.