Escott1377,
300 s.f./ton?? Are you sure? That seems way high, unless you've got an uninsulated, really drafty house. Most installations I've seen run closer to 1,000 s.f./ton, but again this is a wild ass guess, the kind I despise, and can lead to the very sort of problems you mention. Maybe you live in swamp country, but in Michigan my 10 year old house (2200 s.f. with finished basement) has a 3 ton unit and we have no problem maintaining comfort. Most residential units I've seen max out at 5 tons.
FRM,
Hard for us (mostly Americans) to give better advice on oversees brands. Find a reputable installer. He'll carry good brands and hopefully can properly sizethe air in the house. the unit for your needs. Use word of mouth, online reviews, and ask for a list of 5 recent customers.