Replacing Forced air gas furnace

ok so here’s the problem im having. just bought a house and the heater is not blowing very hard at all. call a service guy to look at it. he says the furnace was replaced with a smaller unit then what was original in the house. He thinks originally it had a 5ton unit and now has a 3ton (75000 btu unit). the reasoning behind this the the duct size. he said the duct size was for a larger unit with a much larger blower on it and that is was cheaper to replace the furnace then to redo the duct work.

what i need to know is:

1. is it actually cheaper to put in a new furnace? looked them up and can get a 120K btu unit with 2 fan speeds for about $1500 from lowes or Home Depot.

2. is it super important to match the a/c with the furnace? this is important because the larger a/c evaporator is still in the duct work. they had to make a new connection duct to hook the smaller furnace to the A/C coils.

3. is it better to buy a specific brand or should i just get the most efficient one i can afford?

would really like to get the larger furnace as that would allow me to hook the garage ducting back up so my work shop can be heated and cooled. but if duct work is cheaper….well you know.