Stay away from the bubbles unless you started it )
Buy only the ones that for you, as a business or website owner, would be the best option.
Test them for how they sound, are they confusing, are the words in right order.
Buy 1 or 2 worders, plus created words, but only after you are sure you understand how those patterns work. If you can find good LLLL dot coms for good price, go for it. Yes, mostly stick to dot coms.
Do the simple math regarding profitability of the domains. Because the domain market is not liquid, cash is the king ) Buy cheap when you can and make sure you have enough cash flow to hold on for sales to come. For good quality portfolio at fair pricing you'll be selling 2-3% of your domains a year. And that has to pay for renewals, replenishing/increasing the inventory and profit. So you can't sell for too cheap. Do the math, basically, what the prices have to be.
Many investors also let good names go because they did not get offers in 1-2 years. But, again, only 1 in 20-50 names will be getting an offer in any given year, so that does not mean it is bad.
If you want, send me PM with the sampling of your 50 domains and I will tell you in my opinion which patterns in your investment are worth pursuing more.