There's two ways to go about it.  Spend a lot of money and put in a good quality unit that will last you for years and years and is serviceable, or do like I did and go to Lowe's and buy a whirlpool or whatever they sell now for cheap and just plan on it lasting maybe 10 years and replace the whole thing.  So far it's worked great, I've probably had it 4 or 5 years now.  I think I got the one size bigger than the smallest they had.  They size them for the size of your household I believe.  I'm not sure how long I'll be in this house, so that's why I didn't go with an expensive one.