Juju - My SO and I have had one fit done and are completely happy with them. My SO's fitting resulted in swapping out a few components and nothing more. Mine was more involved.
The guy who did my fitting asked me lots and lots of questions to determine what kind of rider I am (not racing, more the endurance type). He then put me on my bike on a trainer & did lots of measurements, some tweaking and more measurements, then more tweaking. The issues I had were shoulder & neck pain, numbness in my hands & butt pain. In the end, he replaced my saddle, my seatpost & my stem. What I found out is that the top tube of my bike was too long. He brought the bike as close to fitting me as possible, then sent me home to put in a couple hundred miles on it. He wanted me to come back & let him know how it felt. He told me that I might be able to ride with that bike for years to come, but that in the end I would need to get a new bike if I wanted one that fit me properly. Meanwhile he was going to look at stock bicycles & see if any came close to the geometry I needed.
I went home and put a couple hundred miles on the bike while he found a Fuji wsd bike that approximated what needed. I went back to the shop a couple weeks later & tried out the Fuji. What a difference it made to have a bike that fit! However, the Fuji was aluminum and I didn't want an aluminum bike, and it also had 650 wheels & I wanted 700's. He gave me options of manufacturers and in the end I decided to go with an IF custom frame. That was my choice; he never pushed me to go that route. I cannot tell you how wonderful it feels to have a bike that fits correctly!
In the end I think a good fitter will do what he can with what you have, while also being completely honest with you about your equipment. He will leave it up to you to decide what you want to do and how much expense you want to go to. For the record, I am 53 & struggle with weight issues as well.
Good luck and let us know how it goes! -jane
"When I'm on my bike I forget about things like age. I just have fun." Kathy Sessler
2006 Independent Fabrication Custom Ti Crown Jewel (Road, though she has been known to go just about anywhere)/Specialized Jett