The smell is not outrageous, but I am just buying the house and I can tell that in the heat and humidity of summer it is going to be stinky.

I was going to clean, sand and then seal some smelly floors before going over them with new hardwood.
Any tips?
Once guy said a gloss finished locks in smell better.

Actually I really should seal the walls also, I am sheetrocking over the walls and reaplacing the baseboard and flooring, so I can be pretty quick applying a sealant to the walls. I am debating weather a deck sealant in a garden sprayer then rolling would be the quickest.

If the floors are soaked with urine , then probably it's best to sand to bare wood and then re seal. If they are just old and cruddy , you could probably just buff them with 220 grit screen or comparable abrasive and then vacuum and re coat.If you are simply burying the old floors under the new ones , you don[t have to do a good job sanding. Just do one rough cut and then vacuum it and coat with 1 or 2 coats of oil based urethane. It doesn't have to be gloss.

Smelly Smells? wasnt he a rapper in the early 80's?

Yes. Smelly Smells one hit was "Somebody Farted"