Should I buy crafting diagrams from merchants?
As a general rule, the best craft-able gear at any given level will be the green-quality Witcher gear. Witcher armor will have the best resistance to "monster" damage in the game by quite a bit. Also be aware that, for those few times you need it (cough Extreme Cosplay cough), Cat school body armor & boots and Griffin school gloves & pants give unmatched resistance to Elemental damage. So be diligent in purchasing "maps" that lead you on quests to acquire those plans.
One of the main exceptions to this rule is that some non-witcher armor can be found with slightly higher resistances to the slashing/piercing/bludgeoning damage you will receive when facing humanoids. The free armor provided by the master armorer after you activate her is an excellent example of this, and won't require a diagram from you. On my 1st play-through I wasted a fair amount of my carrying capacity lugging around a spare armor set just for fighting humanoids. You should find witcher gear more than up to the task.
You will also be acquiring many low quality rune stones as loot. As such, I purchase on an as-needed basis the diagrams that allow me to combine these lower quality rune stones into their higher quality counterparts because these are diagrams I expect to re-use.
Very late in the game you might outgrow your "master" level Witcher swords. By this time you are very likely to have already looted a level appropriate diagram or weapon. Otherwise, sure, go ahead & buy one...
You will quickly find the crafting interface crowded with useless extra & obsolete diagrams just due to your routine looting. You'll quickly wish they'd included a way to filter out "grey" level or non-Witcher diagrams. There is really no need to spend your cash to accelerate this problem.
As far as I can tell from multiple playthroughs most recipes and diagrams turn up randomly. So you will eventually find the same things in chests - but sometimes it can take a long time (well in to a New Game+ follow on playthrough in the case of one recipe for me).
There are a few diagrams which are only found in one place but I think these are always part of a quest. A lot of the best diagrams (such as the various Witcher sets) are found this way.
So if you have the cash and a merchant has something you particularly like I would buy it, but only if you can use it soon.