My main bit of advice would be: - avoid having to buy (and engineer) things twice.
You buy, for example, an aAspX.
You decide it'll be okay with 5D thrusters and that it'll be okay with a 3A Power Plant, a 4D Power Distributor and a 3A Fuel Scoop
You then fit a 3A shield to it and a 5A collector, and some weapons.
And then you do a bit of engineering on those modules.
Later on, you decide you want more powerful weapons so you buy those and fit them.
You also decide you'd like a stronger shield, a bigger fuel scoop and more powerful thrusters.
Oh dear.
You now find that as a result of fitting a bigger shield, better weapons and more powerful thrusters, your PP isn't big enough and your PDist doesn't supply the amount of juice you need.
As a result of upgrading the shield and fitting a bigger fuel-scoop you also realise you have to remove the 5A collector and replace it with, perhaps, a pair of 3A collectors.
You
now need to travel around to locate the weapons, thrusters, shield and fuel-scoop you want.
You're also going to have to replace the PP and PDist in order to support the new modules.
And then you're going to have to find a bunch more mat's in order to engineer all the new stuff you've fitted.
The best way to avoid all this hassle is to use something like Coriolis.io to plan your builds before you buy anything.
You don't have to buy everything at the same time but make sure the stuff you
do buy is suitable for the finished ship.
In the above case, for example, you might buy an AspX and then fit it with a 4A PP and a 4A PDist even if, to begin with, you fit it with small weapons and D-rated thrusters.
If you plan on fitting a 5A fuel-scoop eventually, even if you buy a 4D fuel-scoop to start with bung it into the C5 slot so that slot is "reserved" for the 5A fuel-scoop when you eventually buy it.
Similar thing with the shield-generator; if you plan on fitting a 6A shield eventually, fit a 6D shield to start with so that it'll be a straight swap for the 6A shield later on.
And, of course, don't waste a lot of mat's engineering modules that are only a temporary fitment on your ship.
Course, you
can store modules which means you could, for example, engineer a 3A shield to optimise it and then store it when you replace it with a 6A shield.
Plan your build and then buy the appropriate modules.