Subfloor and hardwood floor installation:leveling and squeaking
It depends.
I've ripped out subfloors to achieve a level floor so you are in a better position than that.
Floors are generally most level with the supporting members and out of level across them.  Setup a laser and find the high point and then find the low point.  How far out are they?
You can shim the members up in rough fashion to get approximately level then put down plywood ( tng 3/4, subfloor adhesive, screws ).  That is fairly low effort big bang.
After that if you want to get the 1/8&quot; over 10' recommendations you can do SLC.  That's quite a bit more work.
If you don't need nail down you can use underlay. I like the feel of a floor with underlay ( some amount of cushion ) and you get less impact noise.  Some people really like the solid feel and sound of glue or nail down flooring.
I typically go with PL subfloor adhesive though I have used the two part poly urethane foams as well.  Either is fine.
For underlays it depends on the installation.  I have done engineered hardwood overtop of thermal heat transfer plates and hydronic heating and in that case I selected a low-R value to allow the heat to come through the underlay easier.  Since you aren't running a floor heat source I think the only criteria is going to be how compressive the underlay is.  Engineered hardwood typically specifies the maximum compression to be 20 percent and maximum thickness of 1/8&quot;.  This is probably derived from the NWFA ( National Wood Flooring Association ) who are the go to people as far as best practices on flooring installation.  Really I think the point here is that you don't want so much deflection that the boards could slip their connection point.