It is def. NOT uncommon for IP's to start the process w/an agency and matching prior to creating embryos. I've done 5 transfers w/the IP's had to create embryos after we matched. For some, it works out better this way and doing a fresh transfer vs a FET. (frozen embryo transfer) There def. are benefits to creating embryos prior to looking for a surrogate. It can help break up the cost so that it doesn't feel quite so overwhelming to have to come up w/all that $$$ at once. Many agencies will break down their fees and when you deposit them. Such as '$x' due at when you sign a commitment to working w/the agency. Then '$x' amt when you're ready to match and '$x' amt before the transfer takes place. (this is not an exact description, as all agencies are different, but this gives you a rough idea of how it could work) If you want to create embryos prior to matching, it can give you a little bit of extra time, as well as if you choose to have testing done on the embryos, if you end up w/none that are normal or viable, you would have the ability to cycle again for another ER w/out having the surrogate continue to be put on hold.
Your timeline is reasonable, however, there is always the potential for it to take longer. I have a surrogate friend that was working w/her IP's for almost 5yrs before a successful pg. Other's I've seen go 12-15mos. Remembering that surrogacy is all about the hurry up and wait and as much as we hate them, speed bumps happen. Some are just small things that don't really impact the journey while others are more....challenging.
As far as agency involvement, this is something you need to ask for what YOU need. My 1st journey, the agency was super involved and communicated w/both my IP's and myself on a regular basis. They offered to help make appts for me if I wanted them too (that's just too inconvenient for me, so I do all my own), checked in w/us on how we were doing, if we needed anything, they supported us through any/all testing, celebrated the highs w/us and were just wonderful. My 3rd IP's, we did 2 surrogacies. I carried their DD and then their twin boys. We were w/the same agency both times, but we had less involvement, which was what we wanted. (I was very comfortable doing everything on my own after my 1st time, so didn't need the same level of hand-holding as the 1st time. My 3rd IP's wanted more hand-holding for them their 1st time, so the agency focused more on them than me. W/our 2nd surrogacy, we really just wanted them to handle all of the business part of things) We did a fresh transfer where they created the embryos after our contracts were done and then the 2nd journey we did a FET that ended in a BO, so did another ER and created more embryos. There is just under 2yrs between their children and from start to finish, we worked together for 3yrs.
When you find the right agency for you, the right match w/the surrogate and a good attorney, things can move along pretty expediently.