Hi and welcome to here
I have never owned an Aircooled but from 12 years of using a water cooled and reading this forum, I know =
1600 Aircooled = very under powered
(actually I did own one of these, for a week, many years ago, it was slow and blew up in spectacular fashion 
I sold it "as is")
2Ltr Aircooled = a lot of folks like them, reasonable power, can be tuned, a bit thirsty but better if LPG'd
Early 1.9 water cooled (DF) = OK but a bit under powered and prone to expensive problems related to the water jacket to head seals, often resulting in a replacement motor being needed ( ~£1000 )
Later 1.9 water cooled (DG) = (what I have in 2 x vans) = OK all round but also prone to expensive problems related to the water jacket to head seals, often resulting in a replacement motor being needed ( ~£1000 )
2.1 Water cooled, plenty of power, sometimes equipped with fuel injection, which is usually reliable but if it does give problems, there is a lot more to "know about" to fix it. Also prone to throwing con rods if poorly maintained or over revved, plus the usual expensive problems related to the water jacket to head seals, often resulting in a replacement motor being needed ( ~£1000+ ).
Many owners (including me) have an LPG conversion fitted with cuts fuel bills by about 1/3rd (or ~33% for you modern kids
)
Hope that helps.
