The big problem is the power supply. My understanding is that Mexico is 120V 60Hz so your Xbox probably has a 110V power supply, while in Spain the mains is 220V 50 Hz, and your Xbox PSU won't like that too much!
The easiest solution is to take your Xbox with you and buy a replacement PSU when you get to Spain. Before you leave, burn and test copies of Enigmah and Frosty's Rescue Disc to a DVD, so that if your new TV in Spain doesn't support NTSC you can blind switch your Xbox to PAL.
In addition you could backup the E and F partitions on your Xbox disk (I'd do that anyway - twice, to be on the safe side!), and use your current Xbox to put the PAL Splinter Cell gamesaves on your memory card. That way if you can't get a new PSU for your Halo Xbox then you can simply buy a 220V PAL Xbox and a used PAL Splinter Cell when you get to Spain, and mod it all over again. Once your new Xbox is modded you can install a new large disk, and copy the backups onto your new Xbox.