To answer your questions:
- Yes, you always shuffle the deck after searching it.
- If you get a gadget horse, you will have to invent it as normal with a mad scientist, but do so immediately.
- As to whether or not the card is worth it, it's not bad on it's own, but the existence of Maggie Harris makes it a bit obsolete.
Also, if you're running a lot of horses you probably want your 6 of clubs spot free for Calling the Cavalry.
If you don't have those available, however, this is a fine way to get horses out on the board if your deck depends on them, and it fits into your shooting structure. Make sure there's a good reason to include it rather than just including more horses though.