Don't use the metal waterpump,
Use an OE type plastic impellor
WHY????
The OE plastic impellor'd water pumps are an absolute disaster. The impellor shears off the shaft and doesn't spin, resulting in no circulation of coolant.
ALWAYS fit a metal impellor type water pump. Do it right, do it once.
Does it matter whether the parts are the original from SEAT or not?
What i mean is... are the belts for e.g from SEAT more realiable than from somewhere like EuroCarParts??
Volkswagen, Audi, SEAT and Skoda don't make any of the components for your timing belt change. By using OEM parts, you're basically paying for the
badge on the box. Contitech, Gates Powergrip, SKF, Meyle etc are excellent aftermarket manufacturers, one of whom will be making the "genuine" SEAT kit anyway.
Go OEM by all means, but you're paying extra and getting no higher quality than aftermarket suppliers at a fraction of the cost.
As for fitting, find a trusted independant or a trusted local garage. Main dealer labour rates are an absolute joke and you generally find some extremely competent mechanics at small independent garages with a good reputation. I use a local garage who charge £35 per hour labour, they've done cylinder head changes on my 4x4s, suspension work, timing belts, you name it. You'll never find me giving a main dealer £100+ per hour for a spotty teenage "technician" to work on my car.