You're not going to find a spindle capable of 2000-16000 rpm. You're trying to get the best of both worlds, which doesn't exist. You can either get a low speed spindle (<10K RPM), or a high speed spindle (18K-24K RPM), but not both.
You can find plenty of threads where people have built solid machines for cutting aluminum, and all of them are using high speed router spindles.