The 6, 8 and 9.9 horse models were introduced in late 1985 as "New 1986 Models" (they replaced the 7.5 and 9.8 horse models in the line-up).
Between 1986 and midway through 1994 they shared the same block (roughly 12 cubic inches). The carb was different and the exhaust plate was different, all else was the same. So you got the extra horses because the motor could breath "in and out" easier in the higher horse model.
The 8 horse (or even the 6) could be "upped" to 9.9 with a few changes. It is more expensive to do now than it was back when since a couple of the original components needed were changed somewhat, and while they can be adapted to the older motors, it's more work, plus a couple other items that weren't originally required (ie. the original water tube will not fit on the new style exhaust plate, so "both" must be replaced).
In mid-1994 (identified by the "serial", production number) Merc started pairing the 9.9 with the 15 horse, which up until that time had been the only model based on the 16 cubic inch powerhead.
So from the mid-94 model change (which did change the design slightly of a few components), the 6 and 8 retained the 12 cube block and the 9.9 and 15 used the 16 cube block. Once again, a mid-94 up until the last 2 stroke 9.9 built (2012) could be turned into a 15 horse because it has the displacement needed. This "upgrade" is a simple carb swap - the other parts are all identical.
Personally I would look right past the 8 horse unless you get it for such a steal of a price that you can't pass it up.
Of what you have listed my first choice would be the 99, 9.9 because of the displacement advantage - but there is certainly nothing wrong with the 93 model.
I personally own an 86 and 87 model 9.9 as well as a 1991 15 horse - they are all great little motors.
