I find that many LFS lack in up to date reefing / marine information and it is best to seek advice online and through other reef / marine specific forums. If I had ever only gone to buy everything and listen to everything from the lfs id end up broke and in debt trying to buy everything they suggested to deal with this and that, upgrading equipment which they said should be good and in the end Id have a damn fugly reef tank. Puttputt has had one of the best reef tanks in the country with many years of experience, im surprised you didnt take his advice any sooner :facepalm:
Rant over, and back to some sound advice, Id strongly recommend going with a skimmer thats rated for twice your water volume. With a 450L tank, take into account glass thickness, rock, sand volume and displacement you will not end up with 450L. It is hard to overskim, but do not buy two skimmers, thats twice the cleaning and double the amount of things which could go wrong, do it once and do it right.
If you manage to fit in a sump and with a budget of $300 id say go for a NAC6A or better yet, spend an extra $50 and get the NAC7 it is a highly rated skimmer.