Have you gotten in touch with Nicholas Smith Trains (http://www.nicholassmithtrains.com/store/)? They don't build layouts but they have connections everywhere. The store is close to Wayne. Drop by if you haven't. Choo choo barn in Strasburg - same deal. Lance Mindheim (https://lancemindheim.com) builds layouts but not for 3-rail O gauge. But his web sites have tons of info on having a professional layout build and how to design and build your own. Not cheap! Figure around $1,000 per day. Folks who use TrainWorx can spend into the six and seven figures zone.
A 5 1/2 x 10 layout is not too big. You could do a lot yourself, but it depends on which aspects of the hobby you enjoy. For example, there's a thread on the forum now about Mianne benchwork. It's a wonderful system and lots of guys buy it to save building their own. I have the tools and carpentry skills but I still ordered Mianne (it took four months by the way) because I really didn't want to get a bunch of lumber from Home Depot and saw, glue, screw, sand, etc. So, in this case, many of of us pay someone to do some work for us.
If you haven't had a layout in a while, maybe just set up a small loop or two to see what you enjoy. If you pay for a layout based on your current thoughts and experience, you might not enjoy it when it's done. Our tastes change. You can spend enough money on the hobby as it is!
On the other hand, a profession layout builder can help you define and refine your layout so that it will make you happy.
Personally, I'm just not much into scenery, so most of my layouts have none! I like to build the track and wiring infrastructure and run big locomotives. I have a DCC system in addition to Lionel Legacy, and I like fiddling with decoders, etc. So that's what I like. I have track and trains for HO, On30, three-rail O and G which I swap around on my Mianne benchwork depending on my interests at the time. I like the big trains best and I don't have much more space than you do: 9 x 15. I can build an empire in HO, but I generally find the trains too small and the sound feeble. I love the detail though.
I think it's better to have trains than not. If you've got some $$$ to pay someone to help, go for it rather than being stalled at the start.