The Vallejo Urethane Primer that comes in the bigger bottles (not the standard paint-sized dropper bottles, which are literally paints named " primer" that color-match the various primers) is primarily intended for use with an airbrush, but works just fine brushed on. It doesn’t particularly need to be thinned because it’s a urethane paint that basically conforms to details as it dries. It also may not be worth worrying about if it’s streaky or covers fully, as long as there’s a layer of it on the model.
The main purpose of priming is to give your paints something a with a bit more “tooth” than bare plastic to stick to, and even a streaky or thin layer of primer will do that. On the other hand, if you’re painting with very translucent paints, you’re going to want to have an even coat of primer (or gradient of primer if that’s what you’re going for, but that’s a whole other can of worms) underneath so that the tones in your paint aren’t overly affected by the differences in brightness in your primer.
As for rattlecan primer recommendations, I swear by Duplicolor sandable auto primer, which comes in white, black, grey, and rust red. It goes on pretty thin, but a few light coats do a great job of creating a nice surface to paint on. It’s also the stuff that Privateer Press rebadges and doubles the price on for their house brand primer. That said, availability of any aerosol product is going to vary based on where you are in the world, though the fact that you’re being charged $30 for GW White tells me that you’re probably in AU/NZ. That being the case, the recommendation I’ve seen is to hit Bunnings for a can of White Knight Squirts matte (or flat, whichever they call it) in the color you prefer.