The two big server companies are HP and Dell. Some would say IBM and Sun and Supermicro should also be considered for various reasons. I think this may be a good time to go with the big players. The systems are well-known and well-documented on the net. Supermicro I have had a bad experience with, as a server I built for another project *never* got working. (The Cellar briefly tried to make that one a server project y buying a new CPU for it, but it failed.)
The Wikipedia
List of Dell Poweredge Servers page is a great resource. If you go there, we are looking at Generations 8 through 10, and only at rack chassis configurations.
Already it gets complicated. We have a decent amount of rack space to play with - we're unusual that way! - but what makes more sense? Theoretically, we could have a *cluster* of older 1U systems or two younger 2U systems. What requires more maintenance? What can failover well? What runs my choice of Linux best?
That's why the question of our best setup takes time and research and thought.