The biggest threat to Bazaar decks is a restriction, and i don't think it's likely since we have several archetypes that use it - unless something really unforeseeable, it's likely better to restrict around it (imp if dredge actually became broken broken and not regular broken, hollow one if either dredge or survival became a problem, etc)
The financial aspect is too personal for us to guess, i think. You gotta analyze and decide if you have the $4K to spend, or if you can spring for $8k and do something else, or if you want to play an unpowered deck and save many thousands.
You can build a vintage dredge deck for only a nominal amount after you own Bazaars. You really want Lotus and Moxes in Survival but it's easy to branch out into that deck next if you play nonblue (easy is a relative term of course, i just mean it's next step up in terms of how many pieces of power). You coulllld play without the fast mana, haha.
Prices are getting to be to their lowest point of the year with holidays, and they're more likely to rebound or spike than to stay depressed. They certainly won't be their prices they were 2 years ago, or 5 years ago. Prices tend to creep up or spike entering spring.
The alternative to buying Bazaars is to pick up Ancestral and Time Walk and play maybe Jeskai Dreadhorde without fast mana? Or also a pyromancer or delver variant, any list that doesn't really lean on fast mana (like shops and oath for example, which i think are more unplayable without the 6 fast mana than Dreadhorde is without 2 or 3 fast mana).
You can also play Bazaars in a fastbond lands variant and rely on the Mox Diamonds, just play a non-prison version so you don't have to get shops.
Re: where to buy! Facebook is your best bet if you don't mind getting a grasp of it. Prices will always be under SCG/CK/CFB/TCG/Ebay. It's scary to buy from strangers but you can use paypal g&s. I buy and sell tons over FB for side income (i sold $9,958 in October, woot woot) and i could help you track down some pieces and check people's references or straight up 3rd party a sale for you to check for counterfeits. I've done it before and i 3rd party international trades frequently for overseas folks when they run out of luck finding US traders to ship to them.
Other than that, events is great so you can have the card in hand and can negotiate. Ebay auctions can sometimes go great. I have 100 saved searches (the max on the app) for high end cards so i see prices all day every day and sometimes people get lucky.