The first thing I would do is evaluate your current collection. Then if there are issues worth building on, add to them by buying the surrounding issues in same series, in the same grade or higher.

If you tell me the series and issues I already know the answers.
You can also go to Key Collector and see if you have any keys or lack keys in the same series.

80s is a good decade. Right now Marvel Super-Heroes Secret Wars (first series) not Secret Wars II - is getting hot and has room to run. Also it's a good time to buy any Chris Claremont / John Byrne era 80s X-Men. Additionally, the Fantastic Four are back in play and will be for a long time. But it depends what you already have. The 80s also had a good number of miniseries you can pick up cheap and store for long term investment.Some of the best 80s comics were indies too with low print runs. The indy print runs expanded a lot in the 90s as did mostly all comics.