If you buy a current MCS server then you can be pretty sure it will support CUCM8.
However the MCS servers are in a state of transition at the moment. HP, which supply the "H" servers - e.g. MCS-7845H2 - are not happy that Cisco have entered the server market with their Unified Computer System and this seems to be manifesting itself with supply problems and the non-arrival of some platforms such as the MCS-7828-H4 used for CUCM Business Edition.
For the moment I would say your best bet would be to use rebadged IBM servers as the supply situation seems better on these.
In around a year I think we will see Cisco supporting CUCM on their new UCS C-Series rack mount servers. CUCM8 will also introduce support for virtualisation so I expect a fair number of customers to move towards this.
Your timescales mean that you will too early for most of this and that 7.1 is the best way to go (buy UCSS and you can upgrade at no cost when 8 is stable).
If you were doing the project in nine months I would say go for version 8 and maybe look at virtualising it if you have a suitable VMWare environment already in place.
Note that the above is my personal take on things and not the official Cisco stance.
