Flash
For convenience, I would stay away from full manual only flashes like the Yongnuo and others; you don't want to do 'calculations' with aperture, guide numbers etc for every shot. They are however, in general, the cheapest option.
Get a flash with tilt and swivel. There is no absolute need for pTTL if the flash has an auto-thyristor mode.
pTTL models
Pentax AF540FGZ, AF540FGZ/II or AF360FGZ/II
Sigma 610 Super or 610 ST; the Super will be the preferred one
One of the Metz models
From the named ones above, the older Pentax AF540FGZ is most flexible one
auto-thyristor models
Auto-thyristor Flashes | B&H Photo Video
I don't have any of the listed ones, the Vivitar 385HV sounds good but I don't know about build quality
If second hand is an option, look for a Metz 45CT-1 (auto-thyristor) or Metz 45CT-4 (auto-thyristor and full manual with power settings); still using my CT-1 at occasion. There are plenty others, but I'm not familiar with them.
Further add a diffuser to the list of 'what you need' to soften the light.
PS with older flashes, make sure that you buy ones with safe trigger voltages.