1. The speedometer and D4 light flashing is most likely a defective VSS, there are kits available on eBay to fix that.
2. If you have air coming out of the overflow tank then most likely you have a blown head gasket which is an expensive repair. Many people go with a replacement JDM engine instead.
3. These legends are well known for blowing head gaskets, when you get air in the cooling system it will cause more overheating, idling problems, and no heat because there is air by the thermostat and it will not open. FIRST thing to do is bleed the air out of the cooling system.
4. If you are not getting any air coming out of the vents then most likely your blower motor has given up and needs to be replaced. Used ones can be found on eBay and they go for between 50 - 75 plus shipping. The radio code should be in the owners manual, if you don't have that then the only alternative I'm aware of is to call the dealer and see if they will give it to you (some will give it others will charge for it).

For a lot more info, you can also visit www.acura-legend.com which has a lot more Legend specific info and users.