SEV - to buy, or not to buy?

[Suicide Junkie]

I reccommend picking up the package deal where you get SE4 with it. You can't go wrong with 7 years of mods and improvements.

SE5 alone depends at the moment. It is certainly improving continuously, so if you have patience, why not.

As far as colonization, do both. The most militarily useful strategy is generally to rush forward, claiming strategically important chokepoint systems and building resupply depots where nessesary, then backfill everything else as soon as you have a chance.
More colonies, more resources, more shipyards, more everything.
Some mods do allow for more than one level of development on a planet, but generally, you can still develop and expand at the same time without any problems.