1. Get something in writing that provides at least for:
a) Relocation costs to the new location, including the cost of packing and moving your stuff, up to at least a month in temporary accommodation in your current town and/or new location (e.g. if you have to move out of your place before you move to the new location or have to start work in the new location before you have a place to stay), plus travel costs.
b) Relocation costs (similar to the above) if you are relocated within, e.g. 2 years of the move to start up the new location, or your position is eliminated or you're terminated. (This is to help make you whole if the new office location doesn't work out.)
c) A reasonable amount of severance, if they terminate you within a certain amount of time after you move to the new location. (What's reasonable depends on your salary, age, seniority, nature of your position and the job market where you're going, but I'd ask for at least at one year, and have the clause apply if they terminate you within two years of your job start at the new location.
d) A couple of months' job placement service for your spouse, if they will be looking for work in the new location and that kind of service would be useful given the nature of their position.
e) Reimbursement for at least two scouting trips to the new location. (Or if you'll be going to the new location on a work trip, reimbursement to cover the cost of your spouse accompanying you on at least two trips.)
f) If you own a home and decide to rent it instead of selling it, the fees of a rental management company for at least one year. If you decide to sell the house, the contract with your employer should have a "make whole" clause (so that if you sell your house at a loss, they make up the difference). You also should be reimbursed for a reasonable amount of transaction costs associated with selling the house (probably not the commission, but certainly the lawyer's fees).
g) A reasonable number of days associated with the relocation should be paid days off, but not count as vacation days. Depending on the size of your household and the logistics of the move, 3-6 days is probably a good minimum to ask for.
(Ideally, have a lawyer review the written terms they provide.)
2. Make sure that before you make any major commitments or life arrangements regarding the new role, that you see in writing a reasonably detailed plan and budget (with sign-off from appropriately senior management) for the new office location. (For don't sell your house, give up the daycare spot or have your spouse give notice until you see that the new office location is really a go and backed up a budget.)
Note: You probably don't want to spring all of this on them at first. Instead, ask what they're proposing to offer you to make the transition work for both of you. Then consider what you need to add to make the offer make sense. You probably won't get everything you ask for but this list is a start to help you decide what's important.