Hi! I paid my contractor to help me do this near the DC area, and my main couple of issues have been with soundproofing the basement unit from the main house and figuring out the temperature difference between the levels. I'd recommend ensuring that you sound PROOF (not just use dampening material, although of course finding better insulation between levels is great). For example, when testing out noise levels, I can hear my dog's footsteps walking above me (because of his nails touching on the floor) but I can't hear voices or the tv in the main house...so definitely ask around to find the right material! A builder friend recommended 2 layers of drywall on the ceiling between units instead of one that's been great for one of his duplexes, but it's too late for me to add that at this point to mine. I've been shocked how much I love my minisplit AC/heating systems that I had installed into the basement, but not sure what's typical in your area with central air vs. separate systems.
Typically converting a unit into a legal bedrooms takes into account a few different things- square footage requirements, ceiling height minimums, a closet, and an egress. This does vary by state and location, so just double check your county requirements to ensure that no one will give you issues with one of these other items. For example, in the DC area you need 70 sq ft, a closet, an egress, and minimum 7 ft ceilings for most of the bedroom for it to count. (And please, BP community, correct me if I'm wrong!)
I've also found the egress window process to be pretty simple, and it added massive value to my homes- you certainly have the right idea! One tip- make sure that you look at city/county requirements for egress window heights to ensure that it complies with whatever the code dictates. In my area, one county requires that windows must be no higher than 44 inches above the ground, whereas adjacent counties have different requirements. Perhaps making sure the window is located a couple inches below whatever the requirement is would help stave off issues from future code modifications. If it's below ground, you'll need to add a window well with a drainage system (I did this for one of my properties) which cost a lot more, but at my current place it was much cheaper because the basement level was technically at grade and didn't require the window well.
If you're hurting for space in your basement unit kitchen, I'd recommend finding an under the counter washer/dryer. You'll save costs on buying 2 separate machines, and this is pretty common in other countries, particularly Europe. Your tenants will thank you for having it there! I'd also recommend keeping to lighter-colored selections for the basement- you can assume it will already be darker, so lighter paint colors, lighter flooring, white cabinets, lighter counters, lighter backsplash, etc., will make it seem so much more luxurious and less like a basement.
Best of luck, you've got this!!
