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I believe you can call a dealership with your VIN. There will likely be record of it. If not, they could tell by physical inspection.
Sorry to hear about all of these problems. It does sound like the head gasket, and that may be the source of the oil leak, but it may be something else. Regardless, you are going to need a major and expensive repair. My best advice is to have a good technician who really knows Cadillacs check out the car and give you some advice based on it's condition. This doesn't need to be the dealership, and you may save money elsewhere with someone who works on older cars. Check out the common problems page for this car on RepairPal to get an idea of what else could go wrong. The transmission shouldn't develop problems because of the head gasket--they aren't related. http://repairpal.com/cadillac-deville-1998/common_problems
A "tune-up" will not fix this. You may need some regular maintenance, but the reason for the oil leak needs to be fixed. It doesn't sound major, but you need a diagnosis from a competent professional.
You need a new Honda dealer--they should be able to fix this. I would seek another Honda dealership, or a Honda specialist shop with a great reputation. A good technician should be able to figure this out for you.
Take a look at this information on the check engine light: http://repairpal.com/check-engine-light You'll need professional diagnosis and repair.
It is probably the master cylinder. I recommend seeing a professional. This is a safety issue, and it sounds like it is integral to the braking system.
It's probably not the handle itself. It could be the mechanism within the door. You'll likely need some help getting this fixed.
You should be able to add coolant into the radiator. There is an easily accessible cap. If you are still having problems, I would strongly recommend visiting a BMW shop. You don't want to mess around with cooling system problems on a BMW, as if problems exist or persist, you could create massively expensive repairs.