2 thoughts - just because something is cheap does not always make it a good investment
also in investing do not forget your 'gut feeling' = that speedboat set to me just does not seem to be destined to be collectable or special in any way - pretty dull don't you think? (possibly why it is being discontinued)
i don't think there are any stand out friends sets maybe the riding school - they are very bitty , with not obvious long term staying power , IMO. although i do like the ones with azure and lavender pieces like the laboratory and design studio.
(maybe the stage too.)
I think what comes to mind first is a good indicator of staying power .. all about desirability whether right now or 5 years after a set has been retired.
what's desirable now? - go to TRU and watch the kids ...
and anything that's reduced 50% on sale is probably a safe bet though ..
good luck.