I'm planning a ~500 sq.ft DIY install of a 3/8" engineered floor (Bruce Turlington 3", *not* the lock'n'fold) on the entire first floor of my condo (including kitchen, but not bathroom or laundry). I've been reading all over doing my research and I'm pretty confident I'll be able to pull this off with the help of my Dad, who has never done hardwood but is pretty darn handy, and a friend of the family who owns a custom home building co. and has experience with hardwood. That said, I have a couple of questions regarding moisture content/humidity.

I'm located in Pennsylvania, so RH in the house right now is pretty low, probably 20-25%. I'm hoping to get started on the first steps of the project (tearing up the existing flooring, preparing subfloor, etc) towards the end of March (so a little less than a month out), and I'm hoping that the humidity will come up to 30% or so by the time I'm starting to actually install the floor. I'm hoping to order the flooring and have it in the house at least two weeks before I start the project. Does that seem reasonable? I have a basement underneath, mostly unfinished, but one finished room with a drop ceiling. In the summertime I usually keep a dehumidifier running in the basement nonstop (with hose to a drain) plus air conditioner with demand dehumidification, and the RH in the house doesn't get much higher than 55%.

Also, I understand I'll need to purchase a moisture meter to check the flooring and subfloor. Any recommendations on brand, type (probe or probeless), etc? Obviously I don't think I'll need anything super advanced, just something that will get the job done.

Finally, can I check the moisture content of the subfloor before ripping out the existing flooring, or would that be a waste of time? Subfloor is 3/4" plywood, overtop of which I'll be installing a layer of additional underlayment (OSB? 3/8" plywood?) since my joists switch direction halfway across the living room (on either side of a steel i-beam running across the room underneath) and I'd rather not switch the direction of the flooring midway across an open room just to stay perpendicular to the joists

Thanks in advance... so much great info on here, but I'm sure I'll have more questions

I think you should be installing a humidifier for the winter months to reduce the humidity swing from winter to summer. You should try to maintain a 35% humidity at the low end and no more than 55% during the summer.
Also, it has been my experience that when you have a directional change in the joist system, there is usually a "bump" at that location. If so, sand it out berfore installing your plywood underlay.
A really inexpensive moisture metre wont tell you anything other than variations in content. Try to borrow one from someone in the industry if you can, or expect to spend 200 -400 bucks on a decent model. Seems like a waste to me, but perhaps there are metres out there for rent for a day which would be more cost effective.

Would you recommend a humidifier that ties into the HVAC system, or just a standalone whole house humidifer? What about just trying to seal up the house a bit and see if that raises the RH on it's own, since I wouldn't be drawing in so much cold dry air from outside? There are various places around the house that I know I've got cold air coming in (gap between fireplace and surrounding drywall, electrical outlets despite having installed those outlet draft cutout things) and I've also got some ice dam problems, so I'd imagine that means I'm leaking into the attic as well.

I was planning to borrow flooring stapler and finish nailer (for trim work after the floor is down) from the friend who owns the custom home co., so I'll see about a moisture meter too. I'm not too keen on spending several hundred for one. Unrelated to the original topic though, I believe the stapler he has is the Bostitch MIIIFS since they do mostly unfinished 3/4" solid installs and he said it's a Stanley. Now that I think about it, that stapler probably isn't going to work for 3/8" engineered floors is it? I'll need one of the smaller trigger-style staplers?

For your own comfort, as well as the wooden components of your home (including that baby grand) i suggest one built in to your heating system; keeping in mind that the only time it actually is adding humidity to the air is when the furnace is actually running. It can be set to be functional when the fan is running, then you would set the fan to "run" or "on" as opposed to "auto".
Ant the stapler your friend has wont work for your floor. There are several types out there that will though, including cleat types as well as staplers.