I don't think custom would be viable, both from cost perspective and from lack of options (pre-fabricated plates for full size Alps keyboards are either non-existant or very rare, for instance).
You have to hunt around if you want a used board in good condition, but it should be doable (may depend on your market, but here in U.S. it's possible to score a good deal on an Alps board if you aren't looking for a rarer type... may be more challenging in Spain, though).
Buying a Matias is likely the easiest option, and the least likely to have potential headaches due to board or switch condition. I have read of issues with Matias reliability in the past, but those may have been addressed; I don't have a Matias myself, so I can't say from experience.
It's a shame about the lack of 100% size Alps, though. I'm in a similar boat as you. Currently planning a custom Alps build, but decided to settle for 60% and separate numpad due to the limited PCB, plate, and case options.
I've also considered Alps-clone switches and preassembled boards like iRocks K67M (only ~$100 here, and I actually like the LEGO top, as childish as it might be), but wanted to make sure I tried genuine Alps first in order to have a good baseline. I also don't know how good clones are in terms of QA/reliability, though I'm guessing most would be worse than your standard Cherry in that regard.
). My question is, do I get a Matias Tactile Pro or make a custom one? Buying a used keyboard is also an option, but so far couldn't find anything in good condition for my budget (mainly because anything from ebay has no less than 30$ shipping and then the importation tax, rated at 21% of the value). Note that the bigger it is, the better, a tenkeyless like the KB Paradise won't do for me.
