Our family is big with recycling and I think salvaged wood is a good idea. Unfortunately, the price of salvaged wood can often times be more than new wood. Why? It is a lot of labor to remove nailed floorboards in demolition. Also, domestic (USA) hardwood timber/lumber prices are depressed due to the very weak housing and remodeling markets and the flood of cheap wood and associated wood products from overseas. You mention the rustic look, and you really need to define that to see if new wood can meet your objectives. Does rustic to you mean knots or mineral streaks or dents or worm holes or color variations, etc. You might not know it from a trip to a Big Box flooring department, but wood flooring does come in grades other than select. There is also #1 common, #2 common, #3 common, wormy, and other specialty grades and cuts. Each lower grade has features that many would call more rustic and certain features are more common in certain species/grade combinations like mineral in #1 and #2 common red oak. Glancing at current listed prices for USA sourced hardwood flooring on the internet for direct sale to consumer or installer, I see new #1 common unfinished hickory strip at $1.89/sf, new unfinished 2nd grade hard maple 4" wide at $2.39/sf, and new unfinished #1 common black walnut strip at $2.69/sf. Also, I have recently seen new pre-finished select grade red oak strip at $2.50/sf on sale at the local big box store. Clearly, new solid hardwood flooring is very price competitive with almost any flooring product and lasts way longer than the more disposable products like carpet or sheet vinyl. A flooring professional could more clearly answer this question, but uneven mastic on the bottom on boards sounds like a big problem from the beginning as it may interfere with getting tight nailing between subfloor and floorboard. Also, what finishes have been used on these old boards over time? There can be some serious compatibility problems with old wax finishes and newer finishes. Whatever is on there will greatly influence what it may cost to refinish. Finally, the finish will also influence the look as much or more than the wood. Have a flooring professional show you samples of various finishes on the wood you end up choosing. Best of luck with the flooring project.

I suspect that Maple would be about as hard as the Birch and very smooth so it probably won't fare well with the use you're likely to have. 20+ years ago I really wanted a Maple floor until I looked at a kitchen floor we had just completed. The kitchen had a wide pantry door with a wheel supporting the it. Even though it wasn't yet loaded with stuff, the wheel had left a semi circle on the new floor. I quickly realized that if I was to have a Maple floor (and keep it looking nice), I would need to ditch my dog and amend my lifestyle. The most likely product I can think of that would give you the Maple/Birch look and be hard enough to stand the use you'll give it is Armstrong Performance Plus but it's probably going to cost a lot more than your bargain Maple. Right off I can't think of another product that might be in that color range and either be harder or have some texture that would hide dings, dents and scratches. The closest thing might be a natural color Hickory/Pecan which would be harder and have some surface texture but would have way more color variation, even in the nicest grade.

Some time ago I was involved in producing several truckloads of high grade beech flooring in New England. There was some lower quality lumber left over which we ran the into a rustic grade flooring. Some of the employees installed the rustic for themselves and were very pleased. Sorry no pictures, but it did look nice. Beech tends to shrink and swell more than most other species.

Would you consider an engineered floor? I'm no expert but you might be able to have a floating floor installed in which case I think you'd eliminate the need for sleepers unless the floor is horribly uneven. I think the manufacturers suggest specific vapor barriers for their particular product and you might even be able to use some kind of padded underlayment for some insulation?