What size trolling motor for inflatable boat?

18lbs or 30lbs thrust motor will be ample

How fast will a 30 lb thrust trolling motor go?

All trolling motors will push your inflatable kayak or canoe at about 5mph regardless of the thrust

Kayak trolling motor mount ideas

Inflatable kayaks need  firm base for a trolling motor mounting.

Measure and cut a piece of marine ply to cover the existing floor

Screw in the uprights, fit the cross-member (transom bar) and you're in business