The least expensive flash that fits your description is made by Vivitar for $70:
Vivitar DF-293 TTL AF Flash for Pentax Cameras VIVDF293P B&H
If this is for occasional use, and not for demanding conditions, then it's probably good enough. Decades ago, Vivitar and Sunpak were in my budget and so I always used them. The recycle time with this unit is about 7 seconds with alkaline AAs. Usually if you use rechargeable batteries, the flash will recycle faster, but you'll get less discharges, so you will want backups.
However, I've been let down by 'cheap' flash units enough, that now when I am depending on the flash, I've saved up for better units. The least expensive one I'd recommend is this Metz for $220:
Metz mecablitz 44 AF-1 digital Flash for Pentax MZ 44317PS B&H
Metz has made professional-grade flash units since the beginning of time, and this one recycles in a max of 4 seconds (full discharge, max power, alkalines).
Metz mecablitz 44 AF-1 digital Flash for Pentax MZ 44317PS B&H
And then my top choice would be a Pentax flash unit like the AF360FGZ II. It's on sale right now for $343:
Pentax AF360FGZ II Flash 30438 B&H Photo Video
This is the low-end Pentax flash unit, and there are more powerful ones, but the main advantage of the Pentax flash is that like your camera, it's WR and has a max recycle time of 2.5 seconds.
Many on this forum like the bigger sibling Pentax AF540FGZ II and now that it is almost 50% off at $369, that's really a good value:
Pentax AF540FGZ II Flash 30456 B&H Photo Video
So if you need something NOW, I'd go for the Vivitar, but know that you'll outgrow it sooner than later. The Metz is the best real-deal, but won't have WR nor the faster recycle time you want. Personally, I like the smaller Pentax flash unit because of it's compromise of size, power, features, and speed.