How can I trigger - Buying items - that give abilities

Similiar to - The Great Strategy (map)

Id like it so you can pick which abilities you want by buying items.
Im not sure how to do this bit thou.

If a unit already has ability set levelto +1 else add ability.

Could anyone give me some help please.

a unit sells an item

item type of item being sold equal to ....

if level of x abilitiy is less than 1
then
add x ability to buying unit
else
increase level of x ability for buying unit

You'll also need an error section for when they try go increase over the maximum level of the ability.

Also, make the items for sale power ups with a blank model so that they don't need empty inventory space.