Flat-mount to post-mount disc converters

So...the lure of a bargain is strong - but I have both hydro and cable/hydro brakes in post mount (105 and TRP Hy/Rd, respectively)
1. Modern frames are flat-mount - do I just buy some converters and use existing calipers?
2. My winter bike has the hydro discs, but this would be a new gravel bike build. Do I use my nice 105 bits off the winter bike? To be honest, it's a b@ll-ache what with bleeding, etc. and also, the boggo bits off the existing gravel bike are okay, and I wouldn't be upset about wrecking them.


It's just a hill. Get over it.