Well, best bet is to go with newer ones. For example:
Dell R610 (equivalent gen 7) has 2x 4 ocre/8threads CPU and uses 250Wh
HP DL360 gen 8 has 2x 8 core/16 threads CPU and uses 200Wh
HP DL580 gen 7 has 4x 8 core/16 threads CPU and uses 700Wh

As you can see, the gen8 HP has double the core count as Dell and a power usage lower than it.
About the space, there are many ways. If you want just one or 2 servers, you do not need a rack. There are wall mounted supports and it can be wall mounted.