While you can't do a precise load calculation without knowing the actual loads, sometimes you have to make some educated guesses in order to get a project off the ground. We end up doing this quite often -- while one guy is yelling at the Architect, telling him we have to have the actual loads, another guy is starting the design based on a set of standard assumptions.
For example, we typically assume 1500VA for a refrigerator or upright/chest freezer, 1440VA for a countertop microwave, 1800VA for a built-in microwave/range hood, 1/2HP for a garbage disposal, etc. These are numbers that we've found work for us most of the time in the market where we work.
If you have to get started on the load calcs now, and there's no real chance of getting the actual model numbers out of anyone any time soon, you'll have to start making some educated guesses. Start by asking yourself what you know about the Owner/Tenant. Is this a large custom home, or a small apartment? Is there room in the kitchen for a big 2-door refrigerator, or are the cabinets and counters built in such a way that the refrigerator can't be more than 60" tall and 24" wide? Then either go online or drive down to Home Depot and start looking at nameplates. You'll want to be conservative at this stage, so choose the largest loads that you think the owner will be likely to pick and use them in your calculation.
Make sure you tell whoever's in charge (Architect, GC, Owner, etc.) that you're taking this approach, and emphasize that this will all have to be revisited once the actual appliances are selected. It would be a good idea to do this in writing.
Things like the circulating pump and HVAC should be specified by the Mechanical designer. I recommend finding out who that is, and badgering him or her until they give you those loads.
