Phase One is primarily a maker of digital backs for many different medium format cameras, as well as offering some software (Capture One).

Their digital backs have ranged in size from 16MP to 80MP. Earlier ones used CCD sensors, but they were one of the first medium format back makers to convert to better CMOS.

The 645 DF were a 2009 model jointly produced with Mamiya (the same camera sells under that brand, too). I am not certain which of the backs it's able to use. It was superseded by the DF+, not sure exactly when or the differences between them, but that model has since been superseded by the current XF models using "IQ" backs with up to 100MP sensors (approx. 2.5X larger than "full frame" DSLR sensors). Again, I'm not sure of the differences. But the collaboration with Mamiya means that you can likely use a lot of their vintage lenses. Some of the modern lenses are being made in collaboration with Shneider-Kreuznach and feature leaf shutters (one advantage of which is very high flash sync speeds are possible).

I'd wager that 645 DF at KEH for $792 is the body only... no back.

Leaf is another medium format digital back manufacturer that's made backs for the Mamiya cameras. I think Phase One and Leaf have now merged.. and products are selling under both name brands.

I couldn't find a Phase One 645 DF for sale with or without a back....

But a brand new Mamiya Leaf Credo 80MP Digital Back (CCD!) in kit with 645DF Medium Format DSLR and 80mm f/2.8 LS D Lens is selling for $28,495!

Or, brand new Mamiya 645 DF+ body without back but with 80mm lens is selling for $8000. A 50MP Leaf Credo back for it costs $19,995. Aside from the included 80mm, there are eight other lenses available for it (one zoom, seven primes) ranging in price from $2990 to $5490.

In their used dept. B&H has a Mamiya Leaf Credo 40MP Digital Back (only) in Mamiya 645DF mount for $9425. That's for the back alone!

Possibly the "best deal" in MF digital right now is probably the Fuji GFX 50S... a 50MP CMOS mirrorless model with "modern" range of ISOs and 3 frames per second shooting speeds, for $6500 without lens. The back isn't interchangeable and there's fairly few lenses for it yet (one zoom and four primes costing $1500 to $2600 approx.), but I think it can utilize some earlier lenses via adapters (probably manual focus only).

The Pentax 645Z is similar price, specification and design: 51MP, 3 frames per second, non-interchangeable back... except it's NOT mirrorless, uses an optical viewfinder... on sale at both B&H and Adorama for under $5500 right now ($1500 discount). Fifteen Pentax "FA" lenses are available for it (six zooms, nine primes) ranging in price from about $700 (75mm) to nearly $5000 (300mm f/4).

The next least expensive MF digital at B&H is the Hasselblad X1D 50C (also 50MP CMOS, mirrorless, non-interchangeable back) on sale right now for $11,395 with wide angle 45mm f/3.5 lens. Three other XCD lenses for it sell for between $3200 and $4500 apiece. I don't know if other lenses are adaptable to it.

Notice that the least expensive models are all fixed-back. The modular MF such as the 645 DF, in more modern specification offering much higher ISO capability, better image quality, improved AF and frame rates, are A LOT more expensive.

In other words, I think you're dreamin', if you hope to put together a MF digital capable of those incredible images for $1500! You might find some older, low resolution CCD back for little money and fit it to one of the earlier cameras and use them with a vintage, manual focus lens that doesn't cost a fortune. But you'd probably see better overall performance and image quality with a modern full frame DSLR that costs about the same... such as a Canon 6D (older 20MP model) w/24-105mm STM lens is selling for $1700.

If you are not on the wrong forum talking about stick shifts etc., and you are in fact referring to photography and lenses in some way??? . . . prime lenses are not necessarily manual lenses . . . they are still auto focus so not a good comparison to stick shift . . . except . . . prime lenses are like a manual transmission with only one gear. How much fun would the racing car be if it only had a 1st gear and it couldn't shift into 2nd . . or 3rd . . or 4th . . or overdrive??