What make of furnace would fit in the existing hole , duct fed air

Probably not, though there are some considerations; such as the condition of the existing pipework and radiators and whether the new boiler is to be sited in the same (approximate) place as the old boiler.

Companies that fit replacement boilers often prefer to replace an entire system as this not only means the new boiler will perform as expected and "rubber stamp" the warranty but also boost profit. It is difficult for the customer to sort the wheat from the chaff and it would be wise to obtain a qualified second opinion about something that costs a great deal of money.

If you live in a hard water area it is possible the existing pipework and radiators are internally scaled which reduces the flow and efficiency though if a water softener was fitted to the supply and/or if the water temperature never exceeded 140 degrees F, scaling is much less likely.

It will depend on the type of noise.
Most common is the single "Knock" coming from the return air duct "relaxing" after the blower stops.
re: Blower creates a vacuum on the intake side which may slightly collapse the return duct, then it will pop back to its original position when the blower turns off resulting in a popping/knocking sound.
If the offending duct work is exposed and accessible, you may be able to dampen the sound by adding insulation tightly around the duct, or some other means of preventing the duct from collapsing/relaxing during use. Be careful not to reduce or overly restrict air flow through the return duct which may reduce your furnace's performance or shorten blower life.

there must be a crack or even a whole in your air ducts. air ducts are the ones that connects your blow holes to the blower of your a/c. search for the crack and seal it.

also, there are a lot of holes in the river side. holes like holes for the steering wheel, holes for the pedals, holes for the wiring connections. try to cover it. i usually cover holes using foam tape. it is a type of tape that is used in a/c lines. it can absorb water pretty well so no need to replace it regarly.

replace the cabin air filters

blower control module bad. located near blower motor in air duct. held in with 3-7mm screws

There are several options. I like the one that replaces the old heater also, it looks good and gives more BTU for the A/C. But their are some that fit in next to the original heater. Vintage air I think is probably the best unit. Try these places

http://coolfj40.stores.yahoo.net/airconopopfj.html
http://www.cruisercrap.com/vintageair/

Mounted in the air duct. Will be a small plate with a connector going into the duct. When you pull it out will look like wire coils. Many newer vehicles have gone to solid state controls, some are still stuck into the duct for cooling. Wire types rarely failed but connection can get loose.

this is caused by a defective blower motor speed control resistor, it is located near the blower motor in the air duct the blower fits into.
here is a piture of the part. Price about $18

The blower motor resisitor is a small (about 2" x 2") board with (usually 2 ) connectors on it. It fits into a hole in the plenum (ducting) near the blower motor. They are usually under the dash on the passenger side or on the same side under the hood near the firewall. It should come out after removing two screws.

Remove cover under dash on passenger side & look for plug in duct work with wires going in & out of it. usually held in by one or 2 screws. Remove screw(s) and resistor should pull out.