Just had a quick look at their finance deals, and there doesn't seem to be a catch. Remember though that a 10% deposit is required to start the agreement, the remaining balance is 0%, and this includes the delivery charges.
All the computers on offer are basic units which can be customised to your requirements, so no old kit there. Evesham have a very good reputation, and the after-sales support seems to have a very good rating, not like some similar company's.
One other point that I would mention. If you have the unit customised, then you will lose the benefit of the Distance Selling Regulations, but if the unit is built as per Evesham's specifications, then the 7 day rule will apply.
Hope this helps, and welcome to the forum wizza68.