ha, it's easier to buy a car than a pram....carseats aren't much better.
Cots, well it depends on how much you want to spend. If you're in a metro area find a large baby store and test drive. Doesn't mean you have to buy there, but at least you get a feel for the real deal. Internet shopping is hard for stuff lie that.
Know the measurements of your car. No sense buying the fanciest car seat if it doesn't fit. Same with the pram in the boot.
If you see someone random with a pram you like - ask them about it. Most people are happy to help.

With the pram it's easier if you know some basic features you'd like. For example this time I know lots of people recommend a babyjogger pram, but I didn't want a 10kg+ pram. BTDT that and they're a PITA to have to lift repeatedly into a 4WD boot IMO. Parent facing isn't really important to me as I tend to carry my babes until they're about 6mo - but it is important to some. Some people need height adjustable handles due to serious height difference between them and their partners. I live in Central Oz so a HUGE hood is important - many prams don't have this.

There used to be a thread on what feature people found invaluable in prams and what they hated, but the best I can find is this.