Here is one way to think about it.
Let’s say you are considering buying the Lowe for 12,500 gold. Obviously, if you want to buy a premium tank to grind credits, you should also have a premium account (you can do without it but then it will take the prem tank longer to pay for itself), let’s 1 month, which costs 2,500. In total you need 15,000.
If you simply buy credits with this amount (400 credits per 1 gold), you’ll get 6,000,000 credits.
While it’s highly individual but, on average, you should net about 40K credits per match with the Lowe. Which means it will take about 150 matches for Lowe to pay for itself and start earning “profit”.
Think how much you are playing. If you play 30 matches a week, it will take you 5 weeks of playing nothing but Lowe to get those 6 mil. If you play 10 matches a day, it will only take a couple of weeks.
Also see if you have enough time until “end of april” to get the necessary credits before the next patch.
IMO, buying credits with gold saves time but is expensive and is a very short term solution. Buying a prem tank is kind of a long term investment, if you are planning to play for a long time, prem tank will keep paying. 150 fights is nothing to be honest, especially in a good competitive tank. Ever since I bought my Type 59, I played probably close to 500 matches in it, that’s A LOT of millions credits earned.
And don’t forget, you can use a prem tank to train any crew of the same nationality.