I thinking of converting a basic shortboard for kiteboarding, this should save me some money, since fins, and materials, cost $. I figure I should be able to put strap points/inserts in without much work.
I will be only using this board for wave riding, d'ont plan on jumping with it.
Question most shortboards look the same on outline, what should I look for in bottom shape and fin placement when I buy a Cheap used Shortboard.

Good kite surf boards generally have low rocker. If you pick a board with loads of curve from tip to tail it rides slower and is harder to stay upwind. This is usually overlooked but it's the most important characteristic of how the board will ride

Length anywhere from 5'7" to 6'0"
Thickness 2" or less
Width 19"
Construction go Epoxy or some version of it. Regular surfboards will not last very long.

Look for a board without too much rocker, but still some there.
Some pronounced rocker right at the front is good to avoid nose diving when you go to vertical.

Too little tail rocker and you will end up with a "splatty" board.

Look for a bit of concave in the hull, Helps the board track better.

Fin placement depends on the board, but look for something without to much toein as it will cause drag.

You can also reshape your fins a bit. I do this thing called Twisted Fins.
I basicly heat up the fins and bend the tips in so as to reduce the toein on the deeper part of the fin.

Do a search here foe my old twisted fins posting.

For straps you can get NSI's adhesive straps and pads. Very good.

Otherwise you can put some inserts in the board.

But if you are not going for jumps with the surfboard you should try going strapless.
It makes jibing easier, puts less stress on the board and gives you more freedom to move your weight around on the board.

Most people around here on directional ride surfboards rather than kite company directionals.
There is a great used surfboard market here.

Cheers

Fred