400 miles not that far but far enough to have problems.
1. Go through the trailer bearings - clean and new grease. Bearing failure is probably the most likely problem you could run into.
2. Need good trailer tires and yes, a jack and spare are nice to have.
3. Make sure lights on trailer work - don't want to be stopped by some local-yokal wanting to boost his ticket count!
4. Trailer brakes not quite as important but nice to have working.
5. As far as tow vehicle, if you don't have concerns about driving 400 miles without the trailer, you probably don't have any worries about driving with it.
6. I've never strapped my MasterCraft down. The cradle on the trailer is pretty deep and you would have to hit one heck of a bump to bounce it out, but it wouldn't hurt. You should probably use the tie downs that have springs or some kind of stretchy portion to avoid stressing the attachment points from vibrations or small bumps.
7. Along those lines, make sure the strap on your boat winch is good and consider tying a back-up line from your bow to the trailer to keep the boat solidly on the trailer.