So the villain has 5BB and is limping in UTG. Sure he could be one of those players looking to trap with AA or KK by limping, or if he does not have one of those 2 hands, then limping with anything else is a pretty bad play. In general though, I am happy to get it in with top pair at this stage of the game. Here are few things to think about.
What are your options?
. Option 1: You check, he bets (any amount is really pot committing for him at this point). Are you folding here? If so you are saying that he either did limp in with big pockets, or he actually limped in UTG with a hand like A10, K10, etc...With 5 BB if he did limp in with the latter (A10, K10...), then that is a bad play all together. You can then label him as so, in which case I will always be happy to get it in with top pair in a similiar situation when this person limps in, since he is capable of having anything.
. Option 2: Pot is 500, UTG has 850. You bet (say 200 instead of 850), UTG goes all in for 850. So the pot is now 1550, and it costs you another 650 to call. You are getting better than 2:1 on a call here, so you need to call. A worse play would be leading out then folding. My point is, in the situation where you lead out, whether you lead out for all his chips, or a smaller amount like 200, you are going to be all in regardless.
So overall, I'm fine with getting it all in on the flop here (again barring the situation where you know this player would only make this play with AA or KK, but I think at the 1.75 level, you are going to see a lot more players limping in with more hands than premium pockets). And I like leading out here, because it you go check-check, and a J+ falls on the turn, your decision becomes a lot more difficult from here on out. So get it in when you a probably good. What did he end up showing?