First off, find and visit as many churches that have done work of the type you need to do--ask both "who did this?" and "How happy are you with the job they did?" and be prepared for a lot of long conversations--After a while, you will start to see what the options are--

Any building contractor could do the job, but if you want to make structural changes to the building, you may actually want an architect to design and supervise the work. It pays to explore a variety of options--talk with the architect that re-did the building in 1970(if this person can still be found), at least for reference purposes--also, talk to anyone in the church who was involved in that effort--you will learn some interesting things--

Find out as much as you can about the individual procedures that will be involved in the renovation, and what they will cost. i.e.--Drain replacement, plastering--etc. there are books that you can buy which the contractors use to draw up their estimates---you probably don't want to do the contracting yourselves, but this will help you to understand what the contractor is doing, or should be doing--

I have had some remodelling and renovation work done recently, and will be having more done, and my sadder but wiser advice is, the more you know, the better. Don't assume that someone else has got things under control--don't say yes until you understand exactly what is going to be done, and why--

Check this link:
Restoration Trades Catalog