Replacing my existing system

My existing 21-yr old system is on its last leg.
I am told ther may be a leak in the ac***ulator.

If it is replaced, the new system will be a
high-efficiency R-410 system.  My house has 
approximately 1500 sg ft of space to be cooled
and the duct work is in the ceiling.

I have been told:
- there is a substantial price increase between 
  Seer-15 and Seer-16;
- The quality of a Goodman is not significantly
  less than a Carrier or a Trane; and
- 2 tons is undersized, 3 tons is oversized, and
  2.5 tons is just right.

All these statements accurate?

Also my house is brick, built into the side of a
hill, and is west of Ball-i-mer (Baltimore - the 
City that Slurs).  We have oil heat and the A/C
is only used from late May through early September
- plus my wife is a tirant about heating only to
68F and cooling only to 80F.

Dick