90% of the time I have a kit in mind when I go into the hobby shop. If it's there, I pick it up, if not, I'll check with the owner to see if he can get it. If he can, good, if not, I'll check on-line. (I always support my LHS. The privately-owned one uptown is my main kit source, the Hobby Lobby, my tool/supply/material store. Execept during the 40% off coupon week. Then I swap. (I model about 40-50 hours a week, so I'm well-known at both places. I go in each at least twice a week. More if I'm in full-blown diorama mode, like now)
At any rate, price of a kit is uppermost in my mind. I have a firm, non-negotiable 45.00 kit limit, and even then, only certain kits in certain scales are considered. For instance, I'll pay not more than 20.00 for ANY 1/48 single-engine/single-seat aircraft. 35.00 is the max for twins, and 45.00 for 3+ engines. In 1/32nd, it's a solid 35.00, and armor is 1/35th=45.00 max. For kits that run more than my limit, they only get bought if there's a 40%-off coupon that week for Hobby Lobby.
The second thing is manufacturer. I will NOT pay 34.00 bucks for a kit-type from one manufacturer that I can find from another manufacturer for a cheaper price. For instance, the new C-47 from, what is it, Trumpeter? Heard it's retailing for about 80.00 bucks. The Monogram C-47 is less than half the price of that one, it's still out there on Ebay, and there is absolutely nothing wrong with Monogram's kit that some thought, a good parts-bin, and some "creative gizmology" (note I make no reference to after-market) can't fix to make a build every bit as good a "new" kit (. All that interior detail that's so important to some is 99% invisible in that aircraft-type once it's assembled. But that's another thread... Suffice to say that "inferior" kits are kinda my primary choice.
Third is scale. I don't have any flexibility with that, except for special projects. Aircraft are 1/48th exclusively, unless I need to include an aircraft in an armor diorama. Armor is 1/35th only, unless, like above, I need to do a special project that calls for vehicles in an aircraft diorama.
Lastly, impulse. Many times I go in to get specific kits, but another will grab my eye as well, and if it meets all the other criteria, I'll snag it as well...
The exception to all the above is figure sets. I buy those whether I need 'em or not. I "Frankenstein", or kit-bash my figures, so specific poses aren't an issue, but a large selection of body parts is, since I don't build "display" models, only dioramas/vignettes... Aside from seated aircrew and drivers, 99% of my figures are "made" into the poses I want/need.
Kit rarity/collectability are seldom a factor, since I don't collect them, and "rare" kits still have to meet my purchase criteria.