Hmm you're in a tricky spot now. On the one hand, you've clearly done your homework and know what you want. But it's not going to be easy to actually find what you want.. and you're ready to buy now, so waiting is really hard.
I personally would not purchase a pre-owned console from an individual, or an ebay store (unless it's a major chain store just offering the same prices and terms as in-store).
In my area pre-owned consoles from a game store come with all the usual warrenties (granted by the store and by law), and I've actually not heard of anyone having unresolvable issues from purchasing one of those - the stores check each console thoroughly before offering them for sale. Even "cash converter" (you sell your goods for a fraction of the worth, they then sell your goods at a higher price) stores are good about checking consoles first and replacing/refunding if there's any issue (which can always happen, even with a brand new one). In Australia consumer rights are very much protected when it comes to things like this, but I know every country does things differently.
I love my WiiU. It's been fabulous for us and we've not had any issues with it. It was bought new and is now out of warrenty so we're in the currently in the same position as we would be if we'd bought a pre-owned one from someone. If something happens to it, I imagine my partner will try to track down a new or refurbished one just because he's like that, and he got this as a present for me. If he can't find new/reburbished then he'll get a pre-owned one from a local games store. I'd be happy with that to begin with, myself, but it's no big deal either way.
In your position I would look at the re-sell value a WiiU (and it's games) has in your area. Whether you purchase one and decide to sell it again within the next year or so or not, it's important to know what the numbers look like in case that starts looking like the better option as time goes by. Depending on how easy it will be to sell/trade in your WiiU however suits you (going into a store = less money but less hassle, private sale = more money but usually more hassle), the case for a WiiU might look stronger or weaker.
Ultimately you're decision is probably going to come down to a) how long you can wait to play BOTW, b) stock availability of switch and WiiU, and c) which is the better deal for you financially now and in the longer term.
I hope you find a way to get want you want ASAP