If you want to cut lumber, you can get a wood blade, maybe 6 inches long, and that will work for just about anything.
If you want to cut lumber with nails in it, you need a demolition blade which is a combination of wood/metal teeth. I use a demolition blade on my reciprocating saw when I break down some of my pallets. I use a 9 inch demo blade for most of my stuff. But, I am currently looking at getting some 12 inch demo blades because recently I needed a longer demo blade on a project I was doing.
For cutting small trees and branches that you know don't have any metal, such as nails, in them, I use a 9 or 12 inch pruning blade. The pruning blades really cut through tree branches in no time. Many times, for small jobs, I just use the reciprocating saw with the pruning blade and leave my chainsaw in the garage.
If I need to cut metal, then a good 6 inch metal blade usually does the job for me.
The good news is that you can usually find reciprocating saw blade kits that contain all these type of blades. Buying them in a kit can save you lots of money.
Of course, not all blades are made to the same quality. And blades are consumables, they wear out and need to be replaced. Sometimes all I need is an inexpensive blade for a one time job, other times it makes more sense to pay extra for a better-quality blade that lasts much longer and saves you time and money in the longer run.

Anyways, the reciprocating saw is one of the main tools I use all the time, for all kinds of things, because I can easily swap out the blades for the project I am working on. Shoot me a response after you have had your saw for some time and tell me if it was a good purchase for you.