It's been over two years since I made a commentary recommendation. For Wesleyans, I've recommended the NIV Application Commentary series if you are going with a series. But ideally, you would buy commentaries a la carte, one by one.
Anyway, I'm willing to recommend Joel Green's 1997 commentary on Luke as my first choice for Luke, if you want to buy just one Luke commentary.
I'm also willing to recommend Robert Jewett's 2007 Romans commentary as the best one commentary to buy on Romans (although it will put you out a pretty penny).
See you in two years with more... :-)