I'm in a similar type league, we use 100 dollar cap, with a min of 16 roster spots to fill (can fill up to 19 if you have the room). I recommend NOT spending top dollar for Lt and Manning, you can usually save a good amount of money going for the number 2 or 3 guys in the top tier ( say s. jax and T. brady or D. brees instead) this way you have more money for other studs (WR, TE) and also have money for depth.
If you spend 30-40 bucks on LT and he goes down, so does your season.
Best thing I can say is keep track of what EVERY player has left for cap space, also keep track of what they have drafter (if you already 2 QB's, keep naming QB's on your turn to get payers to spend their cap.)
I always do my best to not pay full value for a player. you goal is to try and get 120 bucks worth of players for 100 dollars in cap. guys always slip in the auction. and some guys go for much more than their worth. also keep in mind since its a keeper league, guys like a. Peterson and c. Johnson will go for more than they do in a non keeper format.
do a google search for fantasy football auction strategy, you'll find lots of good info. no 1 strategy works, you have to be flexable. good luck!