Another vote for buying the nicest you can afford, with two caveats:
- make sure in the "afford" equation you include at least some gear to get you riding safely and comfortably right away (do you have a newer helmet? padded shorts? gloves?) and
- most importantly, take some time with a few bikes and really get a sense of how road bikes fit, and how you fit on different road bikes.
A higher end bike that doesn't fit is worse than a 15-year old mountain bike that does! (speaking as someone who rides an 18 y.o. mtb with slicks as my daily commuter, as well as a FANTASTIC custom road bike that took a while to save for!) Good luck!
Sit bones = ~135 mm, saddles that work ~ 155cm/6.1 in wide
2003 da Vinci (custom road/all-rounder)/Terry Butterfly Ti
1994 Gary Fisher Nirvana (vintage MTB/commuter)/Terry Butterfly Chromoly
1991 Terry Symmetry (NOS frame/fork, project in progress)
1973 Raleigh Super Course (project in progress)