I agree with other posters that its probably OK. If the rust is on the feet or outside you are probably OK. Also, as long as it was drained and did not freeze. If its gas, look at the mechanics and make sure they look in reasonable condition. If it has any dents it could be trouble because the lining could be busted up. if no dents then you are good to go.
If your really anal, you can replace the dip tube(s), pressure relief valve and washer on the drain. Flush out with clean water first, if full of gunk/rust then get a new one. If it runs clear pretty quickly then go for it. Install it with a pan plumbed to drain or sump if a leak will cause issues. Whats the worst that can happen?