It is too late at night for me to want to volunteer for the job, but yes, I think 'buy ahead' is absolutely a candidate for Dubious Entry.
(To answer your initial question, it would absolutely have to be 'buy sth. ahead (of time),' never 'buy *ahead sth.' So it's evidently not actually a separable verb, but just a verb with an adverbial phrase that can be shortened to a single adverb?)
I have at least collected some dictionary entries for 'stock up (on),' in case that helps.
There may be some BE/AE difference, since a BE dictionary lists 'stock' as a synonym without 'up,' and gives an example of 'stock up with' (instead of 'on'), which in my experience isn't as idiomatic.
(In AE, 'stock' without 'up' is transitive and needs an object, as in 'The grocery store needs to stock more cleaning supplies,' or 'We need to stock Mom's freezer with casseroles.')
stock up – to get a large quantity of something for later use —often + on
We made sure to stock up on food before the storm hit.
stock – ... (v. intr.) 1. To gather and lay in a supply of something:
stock up on canned goods.
stock up – 1. stock [sense 5]
2. PHRASAL VERB
If you stock up on something, you buy a lot of it, in case you cannot get it later.
The authorities have urged people to stock up on fuel. [VP + on/with]
New Yorkers have been stocking up with bottled water.[VPon/with n]
same as stock
We’ve stocked up with food in case we get snowed in.
stock – ...
stock something up –
to fill something with goods, food, etc.
We need to stock up the freezer.
stock up (on/with something) –
to buy a lot of something so that you can use it later
We ought to stock up on sun cream before our trip.
stock up –
to buy a lot of something in order to keep it for when you need to use it later
I have to stock up on snacks for the party.