There's no special need to buy so many ovens
When you master all recipes, the profit you will get from selling them should cover the cost of ovens:
Santa's oven: mastery profit $618K. Decrease $40K for each oven - you will get how much profit they will give you.
These two are worse:
World games: mastery profit $331K - minus $50K for each oven.
Healthy sweets: mastery profit $381K - minus $50K for each oven.
(4 ovens each - and profit is less than cost)
And Marzipan - it only has 1 recipe, mastery profit $80K. 2 ovens sd be enough to cover their cost and to stay in plus
Of course, you can keep doing recipes after you've mastered them, but only imagine how many of them you'll have to cook just to get back your money for buying 18 of them!
))
As to disappearance: ovens can of course disappear from the market. But once you've bought the box - it's yours forever. Not only can you cook and master later - you can build them later! Once the box is in your bakery - you can ask for parts whenever you want, when you even cannot buy more coz they're removed from the market.
The only thing you sd take into account- how many free slots you have left for making usual goals.
Because you cannot store the box - only fully built oven from it. Box you can either keep out until built or sell - but if you sell the box, the oven is gone and you cannot buy it anymore if it's not on the market.
Appliances profitability info, tap show after spoiler: