Find somebody who knows about XT500`S, whether it be your local Honda dealer or via a forum
These might be a good start;-

Yamaha Thumpers :: Home
XT500 - UK

Ask if there are any common problems with first gear and if anyone knows what the likely cause is and what needs to be done to sort it i.e. is it a DIY job or something for a mechanic. Ask how long it should take a mechanic. At least then you will get an idea what it will cost to fix.

You could just set off without a first gear. Being a trailee the second gear will be fairly high anyway so easy to pull away with a bit more deft use of the clutch.

As people have said, for 300 of your bucks you can`t go wrong.

Dudes! I Got It
 

I got my bike and it starts right up! The tank and exhaust are rusted, but it runs real powerful and starting in second is no problem. Now to make it street legal!