What to know before you even consider pressing the "purchase" button
Blitz used to be straight forward with it's pricing. A premium tank was only to buy for gold in the tech tree, for fixed amounts of gold, for example the T34, an American heavy tank for 7600 gold. This used to be a much heftier price back in 2016 compared to now. Gold used to be a currency you could only buy. Now everyone gets 50 gold free every day and events grant some free gold as well. Which also explains why new tanks are outrageous priced nowadays.
Another consequence of this pattern is that tech tree premium tanks are not as popular anymore. WG tries to combat this by giving huge discounts on tech tree premiums with all equipment slots unlocked, including things like a legendary camouflage and an avatar. A couple of days ago (it's now June 1, 2020) there was an offer in store for the T34, priced 5500 gold for all the things I mentioned earlier. This leads me to tip 1:
#1 NEVER buy tech tree premium tanks at full price
Every tank in the tech tree will come back sooner or later for a much better deal than you can possibly get from the barebones tank. It's important to be a patient tanker! I mentioned the fact that WG often puts huge discounts tanks. While this is technically true, don't believe it (entirely). WG has decided that 1 gold = 400 credits, which means equipment slots are given huge value. Credits are easily earned by playing though which means the value for yourself is much lower. Personally I don't value the slots at any gold or money, simply because I never have a shortage of money. It's still better to get them and pay the same price of course, that's obvious.
#2 Equipment slots are worth no gold (400 silver for 1 gold is a scam)
What about people that have no time to grind credits themselves? If you play enough to want a premium tanks you probably have enough credits to equip your tanks. This might be an issue for your first premium of course, which is why you might value the free equipment slots higher. That's perfectly normal. Just don't forget that the equipment slots are valued in gold by WG: it's important to calculate the actual savings yourself.
#3 Don't buy crates
Crates are bad value. If you don't have the money for a whole collection of premium tanks you should never buy crates. If you REALLY must, consider setting a limit and take your losses if you end up with nothing. What I personally do is usually buy the cheap crate deal (1+1 free), certainly if it contains gold. Sometimes I'm lucky and get a cheap tank. In most cases I get more or less my value in gold back. In both cases I didn't lose a thing. Buying a truckload of crates is only a 'good' idea if you have unlimited money to pour into the game. And in that case you don't really need purchasing advice, don't you?
#4 You don't need the 'best' tank to perform well or earn credits
WG is not stupid. They know very well what tanks are perceived in the community as OP or very nice to have. These tanks will get a huge price or put in crates. If you have a limited budget, you should stay away from these offers. Yes, an Object 252U is better than an IS-5, but really not by much. The latter only costs you 1,5k gold, which makes it a much better offer. Except for a couple of very OP tanks like the Chinese Fake Tank or the Dracula, the better tanks also decrease in price eventually. Which leads me to the next tip:
#5 Don't buy new tanks for the initial offer
Unless WG makes a stupid mistake (for example Smasher or Keni Otsu), tanks won't get any more expensive. It's thus logical to wait a bit after an intial offer. Tanks like AMX M4 49 were introduced at a 17k gold price point, which is ridiculous. The last offer for this tank was 7,5k gold. The AMX is a very capable tank and certainly above average. It pays to have some patience! Also, if your budget is really tight but you want a nice premium tank to have fun and earn some money, it's perfectly fine to buy a tank lower than tier 8. If you're a beginner it makes sense to NOT buy a high tier tank anyway. Don't let people scare you away from tier 5 or 6 premiums. They make money and are much cheaper, and are better suited for someone who isn't ready yet for higher tiers.
#6 Mid-tier premium tanks (tier 5-7) ARE worth it
Keep in mind that WG offers lower tier tanks for low prices very often (1-2 times/ week). There are also sometimes sales if you're not interested in the padding WG includes in every offer. A tier V premium tank from the tech tree like the T14 is worth it for 1200 gold! Stay away from the tier 1-3 and most tier 4 premium and collectors though: they are mostly UP, don't make money and are not even fun to play.