If I’m installing and reselling $25k of professional sound equipment in a church, what major terms need to be mentioned in a contract to protect my LLC?

You really want to let an attorney draft this for you--there are *many* terms you want to nail down. If you insist on doing this yourself, some of the main ones:
1) Amount of payment and when is payment due;
2) Can the customer cancel or return the order and get a refund (your choice: you only have to refund if you can't comply, not if the customer pulls out);
3) What happens in the event of non-payment or late payment--can you get late fees or interest; can you charge them your legal fees to collect; can your retrieve (repossess) the equipment;
4) That you are entitled to rely on what the customer tells you about their premises and needs, and are not liable if they gave you the wrong information;
5) That you not liable if damage is done to the equipment due to their infrastructure (like bad wiring); 
6) That you are not liable for defects in equipment manufactured by others;
7) Is a deposit required, or is all payment due after te work is done;
8) In the event of a dispute, have the case heard by the court(s) in your county;
9) Where are official notices sent, and how (e.g. to what address, by what means).
Those are some of the major points to include in the contract, but there are more, too. Think about all contingencies--what could happen? what could go wrong?--and write up terms to address them.