Actually, that's a pretty decent question. From my construction days, the quote process could become pretty involved.
I would think it should have an expiration date on it. Also, it should have criteria for you performing the work.
For example, when I would quote calibration work, I would always need to include having ready access to the equipment. If it was a high temperature furnace, it would need to be cooled down. I would need to also note if I expected them to have a forklift available, did they provide a lift (or was I), etc.
Depending on your line of work, I would think a simple checklist to make sure all the basis are covered. Even if you're just selling equipment, noting what is included (not included), warranty, extended warranty, etc. might be things that should be addressed.
Just some thoughts.