question 1 : A cat is indeed more suitable than for you intended goals than a dinghy. A small keelboat will do better although a cat will give you more excitment. A Keel boat is leasuyre sailing and a cat is sportive sailing.
Question 2 : In broad terms all you say makes sense. With respect to singlehander, doublehander, third person I must say that that is very tough condition to satisfy well. A keelboat may well be better in this versatility than all other boats. Problem with keelboats however is that they get rather rapidly cumbersom with increasing size (3 persons) and will require bigger investments in time and money.
With respect to a cat the condition to singlehand and doublehand properly is something a few boats do offer but non of them with combine singlehand and triple hand well. So I would advice you to give priority to one side of the spectrum :single-double hand; double-triple hande. and accept that the other side of the spectrum is not really practical. Some will tell you that some boat do all well, but that is misleading at best. A good singlehander puts a set of requirements on the boat that a triple hander would not and vica versa. The two roles are often in conflict with eachother. Example, A triple hander needs to be more robust and heavier with more volume and more sailpower. A singlenhander needs to rather lighter than heavier and limted in sailpower to be safe to sail and to be able to right her after capsize.
Question 3 : rick white cat sailing for the 90's;
Question 4 ; if your handy yes
Boat typs ein order of affordability :
Singelhand-double hand :
Prindle 16
Hobie 16
Nacra 5.0
Dart 18
Mystere 5.0
... (leap in cost) ...
F16's (Taipans, Stealths, Bim F16's, Ventilo Zipo 16)
Doublehander - triple hander :
Prindle 18
Hobie 18
Nacra 5.5
Mystere 5.5
...(leap in cost)...
F18HT's (Bim Javelin 2, Ventilo F18HT)
F18's (Tigers, Nacra inetr18 Nacra F18, Mystere Twister)
I hope this helps
Wouter