Hello all. We just bought a new house.

1. First floor has hardwood floors(oak) but they had dogs that they let pee on the floors and never cleaned it up. We've used baking soda and vinegar and it seems to be getting the smell up. Hopefully we are able to refinish and don't have to rip them all up and pay new ones. Will sanding them down remove the smell?

2. Upstairs ripped up all the flooring down to sub floor. Going to install new hard wood flooring.

Is laying hardwood floor a very tough DIY project? I'm handy and good with tools. Thanks a lot for nay feedback.

Urine will stain the flooring up to a quarter inch deep, so sanding it out completely is not practical. Sanding the surface can lessen the smell, but if you get close to the bare wood it still smells. A solvent based finish will help seal the smell, like universal sealer dewaxed shellac. Then you can finish with poly-urethane. I prefer to stain the floor a dark color like walnut so the dark urine stains don't show up so badly. You might consider covering the floor with 6 mil plastic to seal the smell and laying a laminate floor if you find one that looks nice.
Laying a new floor over a sub-floor is a lot of work. You may want to get a book on finish carpentry so you can understand the details involved. Renting a stapler and using refinished flooring is the best value. Get the color that you like and you will never need to sand to bare wood to refinish. Prefinished flooring has finish that is better than what you can buy in many stores and holds up well. Add another coat of finish in 30 years, if you take care of it. Don't worry about how thick the surface is. If you are not going to lay flooring over the stairs I would stay with 3/8ths inch thick, so you will still keep you steps up to code. A power miter box saw, air compressor and finish nailer will do most of the work. An electric planer will help scribe fit to the walls. With engineered flooring you don't usually need to add extra trim against the walls. Take it one board at a time as you get started.

Installing hardwood flooring is a very doable. It will be easier if you have the right tools. Also, it's a good idea to do some reading so you have a good feel for the process. Charles Peterson wrote a very good book on the subject. In addition to explaining how he also explains why.
Prices for flooring nailers run the gamut pricewise. You could rent this tool but depending upon how fast you work it may turn out to cost more than buying your own. Probably the least expensive is the Harbor Freight ($99 on sale) which is a diamond-in-the-rough. You could also buy a mid-priced nailer and sell it when done. Compressor......don't need a large one and the nailing pressure is around 80 psi for flooring nailing. Miter saw will save a lot of time. Oscillating multi tool for undercutting door jambs. Router will allow you to create tongues or grooves on the end of flooring if you have to cut longer pieces to length. Table saw would be handy if you have to rip down boards or have to make your own nosing at the top of the stairs.
Good idea to do your home work in so far as flooring layout e.g. where to start, etc.
Also, if you haven't decided on your floor style, brand, etc spend time doing the homework.