Idiom for someone who buys all the best gear to do something before they even have a basic proficiency?

In British English there is a common colloquialism:

"All the gear, no idea"

It describes your requirements perfectly: novices who splash out on expensive equipment but who lack the aptitude to use it properly or even to perform satisfactorily in the subject endeavour.

Unfortunately, I can't find a reputable reference work to back up my claim but, as a native Brit and current UK resident, I can assure you that this phrase is well established and popularly understood.

However, I can provide some evidence that the phrase is in use by quoting this Telegraph newspaper article.

In it, the author (a Telegraph journalist) recounts his first experience on a racing yacht (emphasis mine).

Well out of my comfort zone, even my attire was a giveaway – brand-new thermals and squeaky-clean boat shoes. The phrase “all the gear, no idea” sprang to mind.

[...]

“Have you sailed before?” asked my skipper, Emily, as I clung to the guardrail wire.

“I lived on a narrowboat last summer,” I replied. “Does that count?”

“Not really,” she said with a smile.

The Telegraph is a well-regarded newspaper in the UK, so the phrases it publishes could certainly be considered acceptable, commonly-used English.

EDIT: Even the venerable BBC seems to use the phrase, as seen in a recent article about cycling (emphasis mine):

Yes, there are the oft-derided "middle-aged-men in Lycra" spending their disposable on flashy steeds instead of sports cars and motorbikes. Sneered at by the old guard for having "all the gear, no idea".

The urban dictionary gives an accurate (if poorly written) definition:

This is when you see people (mainly middle aged men) walking around an amateur golf range with all the equipment that has probably set them back a small fortune, however, they haven't even played before and [are] completely useless, yet they have still spent all that money.

In America, the slang is "All hat and no cattle", referring to a (Texas) cowboy with a big hat but no ranch. It extends to people who talk "big" but have no real experience.