Suggestions for basic jazz piano gigging setup

Bob-

Rather than just asking for advice, it seems you need to do some research on your own first to understand the issues you're facing a bit better. No disrespect intended, but your question doesn't provide enough information to really be answerable as written, which is why it hasn't received much response so far.

Does your intended gigging use also require you to provide amplification? If so, then your budget is quite limited and will basically go toward that. Since you already have the keyboard you intend to use, you'll need to decide on your speaker/amplification next, especially since you don't seem too picky about your piano sound. Your amplification needs (and therefore price) will be determined by the kinds of gigs you will be doing (e.g. size of room, indoor/outdoor, solo piano in a quiet space vs. with a band in a louder room).

At $600, assuming you don't need something battery powered for busking (but those are also available...), you are likely looking at a single powered speaker like a QSC CP8, an Electro Voice ZX-A1 or QSC K8.2 (I have purchased a demos on sale for around that price in the past). There are cheaper models if necessary as well. The portable line arrays are also coming down in price and are interesting if you don't need a lot of volume; this one is one of the cheaper ones: https://www.sweetwater.com/store/detail...e-ip500-v2

You could connect your iPad to one of those powered speakers directly (i.e. without going through an audio interface) via an 1/8" stereo audio cable, using any iOS piano sound to try it out. Try it with the free GarageBand Steinway first, then other iOS pianos like the Ravenscroft, one of the pianos with Korg Module running Synthogy, or Pure Piano. That may be enough for you?

If you want to use Pianoteq, I'd wonder if you have an old computer lying around to use? It doesn't take much processing power.  There are some threads here on the forum using Raspberry Pi's, which are quite inexpensive. Someday, hopefully soon, Modartt will come out with Pianoteq on iOS. They said last summer they were working on Pianoteq for iOS, so maybe we'll see it this year. You can't count on it for now, but it's coming. Someday...

FYI, The Steinway D jazz preset for Pianoteq 4 was modified by Modartt in one of their old workshops to get a Bill Evans trio ca. 1965-style sound:
https://www.modartt.com/index.php?page=...tion=evans

There are FXP files from that workshop available on that workshop page, building off the Steinway D jazz preset in Pianoteq 4; there are also other Bill Evans-like files available in FXP Corner.

This: 

I have experimented with a bunch and always go back to decent PA speakers, such as QSC k8 -- small, clean, powerful

Sorry if my suggestion is a little late but I do use Pianoteq on jazz gigs.
I have to recommend a MacBook over a Windows machine. I'm using, still, a 2012 i5 and it is a blast. Plenty of power for Pianoteq standalone which runs flawlessly while Logic X is playing bass and drum tracks. Should work the same with GarageBand. This way, when you are changing accompaniment, the piano is still live.

The set up is still fast enough to add a couple or 3 AudioModeling tracks (I'm using the bass almost exclusively for jazz tracks) and plenty of sampled tracks using EXS24 or fewer with Kontakt libraries.

Good luck.