That makes perfect sense. I used to work for a company called Overclockers UK - they sell components as well as full custom builds. They have some configurator options on their site, where you can basically pick the components you want for the PC, or you can call them and discuss full custom builds for anything from a very basic office PC to a high end gaming rig - they'll build whatever you want, within reason.
Building your own will work out a little cheaper, but perhaps taking shipping into consideration the difference may not be much - also, you'll get a warranty on the full system rather than individual components if you buy a prebuilt machine.
Obviously, Overclockers aren't your only option, I only mention them because I know how they operate, their prices and service are good, and I know they will ship to Tenerife. They'd also be happy to leave the appropriate power connectors, etc free and in place for you to install your HDDs and bits yourself when it arrives - at least, I know I did that several times while I was there.
I'd advise having a look around on their website to get an idea of what's around, and maybe give them a call to discuss options, see what kind of system you can build up, and post back with your findings.
I'll have a look myself this afternoon/evening and see what I can come up with for you, too.
