I can definitely speak to this one...
I own 4 cabins - 2 1BRs, a 7 BR and an 8 BR. Three of these were bought from @Avery Carl who is awesome and will steer you right. I hope/expect to buy another couple this year, too.
1. Age of the cabin isn't a huge factor in booking and probably less important than a normal SFH or LTR in your home town. People look for the amenities, the reviews, the location and the price. Age of the cabin isn't on the listing and people won't even know. As with any new construction, you'll have a year of shakeout where you find things that aren't quite right -- a plumbing leak, a window seal, etc. But at the same time you'll have all new big ticket items - AC, water heater, roof, stain, etc. Older cabins will have had time to settle and any foundation issues should be easy to spot which is a bonus. New cabins should have a builder warranty. In my mind it's a bit of a wash, all things being equal. (I'd just be careful getting cabins that can feel really dated like some of the ugly A frames in Gatlinburg, the heart shaped jacuzzi tubs, etc)
2. A real estate mentor of mine told me that I should go for the biggest cabin I could afford, so that's what made me start with a 7BR. That cabin should see upwards of 130k this year and product $3700/m in cash flow, even with the down 6ish weeks of COVID. My 1BR cabins will both see around 45k and throw off is around $1200/m after all expenses. If you factor in big expenses that didn't hit the small ones, the large cabins should have a better COC return over time.
As for bookings...
I average over 90% occupancy year round across all the cabins I own/manage. Jan/Feb are slow but the rest of the year is generally ok. During peak COVID I was still up around 75% booked while the area was under 10% booked for over 6 weeks. (I've written programs to do data analysis of the local cabin mgmt companies as well as airbnb/vrbo so I can data mine it)
I never, ever worry about my small cabins being booked. Even up to 3 and 4 BR you will book very well for families that want a weekend getaway. Big cabins tend to book further in advance and definitely felt the downturn more than the small ones. But it's so nice to get a 9k Christmas week booking in July...
That said, my booty is always puckered on checkout day for the big cabins wondering what sort of mess the guests left. While the smaller cabins can also get messy, a couple or family rarely trashes the place like a group of 5 families sharing a place. If you're paying a PM it doesn't matter MUCH, but every now and then expect a bill when an oven door gets ripped off (2 months ago) or something similar.
Regarding PMs, I've worked with two local companies that charged 25/30%. One fired me as a client when I was booking more stays than they were (and not paying their commission, of course). The other I fired after I purchased because I could do better than the 90k in bookings they already had for the year.
While you can definitely manage yourself, PMs can do a better job for a few things: on the ground help for guests (removing you from phone calls), setting pricing (maximizing your earnings) and maintaining your investment (keeping it in top shape). Since this is your investment, be sure to get regular updates and stay on top of it.
Let me share you an example from a client I just brought on board this week. He has a beautiful new 3BR cabin in Gatlinburg. From Jan 1-May 31 he brought in $8,000 in bookings using Evolve Vacation Rentals. Both of my 1BR cabins brought in $9,000 in bookings during that same time frame. Evolve wasn't squeaking out every dollar for FIVE MONTHS of bookings because they're a big company and don't take a personal interest. Be sure to ask those hard questions should you go the PM route.
Happy to geek out about this stuff if you have questions. I know Luke is happy to share all the specifics on the self-managed side, too.