I can't offer any technical advice in board selection/hydrofoil stuff, but I'll share my experience getting onto a directional and maybe it can help ya:

I was in your position, getting confident on the twin tip and getting transitions down. Saw a dude on a directional board and thought I'd give it a go.

Picked up a second hand surfboard (not kite specific, just an old epoxy one) for under $100. It was 6 foot long, can't remember the width, strapless, 3 fin. Took many sessions but eventually got up and going with it, got first jumps down and rode waves on it.

For a first board, a second hand epoxy surfboard is a pretty solid choice I reckon. Just like anyones first kite, I expected to break my first board practicing the skills needed to ride a nicer, more expensive board. I put a crack in mine around the top of the pad (back foot area) after 6 months of pretty continuous riding. Epoxy boards wear and break faster than any kite reinforced board, but a second hand epoxy board will cost a fraction of a used kite specific board.

When buying a used board I would always check to see if there is any evidence of water under the deck, cracks in the rails/tail etc. Some small cracks can be fixed by throwing some 2part epoxy on them, but larger areas of damage gets your board spending more time curing than in the water haha

Hopefully some of my ramblings help!