I agree that a first gen would be my choice to hop up/ride/beat on, as there are plenty of them to be had, not so many 86's even fewer 87's, I would leave those as original as possible (unless they've already been beat to hell). As for performance, the 86-87 are a much better design, good ride quality, better frame, better kicker, and the 6th gear is nice too. But really it's not a world of difference, I own a couple 1st gen. and my riding buddy has an '86. His is bone stock, mine has minor upgrades, nothing extreme, piped, jetted, cam, played with sprocket sizes, etc. Through trails we're pretty even (I know alot depends on rider) drag racing mine is faster unless it's far enough for him to run in sixth, and of course he'll take me in top speed.
Enjoy your '84, freshen up the motor and get it running how you'd like, plenty of threads on here about hop ups, the sky's the limit. Once your happy with performance, another $200 worth of maier plastics and a seat cover will have it looking damn good! AND...post up some pics of as you go...
'74 ATC70 (stock for my nieces)
'84 ATC70 big carb, race cam, and piped (big boys little toy)
'84 ATC125M
'85 ATC125M
'85 ATC185S (200X carb and pipe)
'83 ATC200X (my first trike ever, avatar pic)
'85 ATC200x
'85.5 Tri-Z 250
'85 ATC250R
'79 CJ7 w/351 Cleveland
'77 XL75, '78 XL125
'88 Katana 1100
'00 King quad
'05 LTZ400