It is possible to use 645 and 67 lenses on any K mount 35mm body using the appropriate pentax adapter.
You don't need any particulat spec of body to use a telephoto lens on it, but my particular favourites in terms of ergonomics and features are the MZ5n and MZ3. These both have traditional style shutter dials and mix well with my Pentax MX - it is possible to use both old and new cameras in pretty much the same way.
If you use the 645 then the MZ5n and MZ3 also operate in much the same way as the 645n and there is little difficulty using the two formats side by side. This consistency of operation has kept me using Pentax cameras and its convenience is not to be underestimated.
Using medium format lenses on 35mm bodies is OK, but a downside that I see is that the lenses will be very much bigger than the 35mm equivalents. This may or may not be a factor to consider. Also, if you only had one lens you wanted to use, then it would be as cheap to buy a 35mm format lens as an adapter (or very close).
Another thought: if the objective is to use the greater magnification that will result in putting the lens on a 35mm body why not just enlarge the centre of the 645 negative? I would actually cut the negative to the proportion I wanted and then use it in my 35mm enlarger.
Hope these thoughts are helpful to you.
Best wishes

Just to expand on my previous post, the following accessories for the MX and LX could conceivably be of use for the nature photographer:

Bulk Film Magazine LX
Bulk Film Magazine MX (both up to 250 exposures)
Infra Red Remote Control System with three channel IR transmitter and IR receiver (operation of up to 3 cameras or flash units up to 60 metres away)
Wired Remote Control System (masses of wiring and power supply options)
Power Pack M (timer and power supply unit using AC mains)
Power Cords and Trigger Cords (Various)
Motor Drive LX
Motor Drive MX (both up to 5fps)
Winder LX
Winder MX (both up to 2fps)

I've never seen these items, but details can be found in the "Pentax Lenses and Accessories" booklets from the appropriate periods.

The MZ5n and MZ3 are both current cameras that have built in motors, both of which give a distinct buzz when winding. The MX can only be found second hand and has a manual wind lever, so that is silent! However, if speed is needed the MX can be fitted with the motor drive for a full 5 frames per second - if you can find a second hand motor drive. I doubt if this is particularly quiet, but I have never heard one so I can't be sure. The modern cameras are not as fast as this, perhaps 1.5 or 2 frames per second.
The MZ series have electronic cable releases. The MX has a traditional release socket and also had all sorts of remote equipment for it - the only difficulty would be the rarity on the second hand market of these unusual accessories.
Despite having several camera bodies, it's the MX that gets used the most, but that's just my preference for a metal camera that feels solid in the hand.
I'll bet that 500mm lens is a beauty!
Best wishes,

I read this with a strange sense of deja vu - I seem to have just replied to this in the lenses section........

See you there!

I have had several ME series cameras - ME, ME Super, MV, MV1 and even the LX which have all,eventually, had similar electronics problems. All easily fixable, but it seems the electronics are the first things to go wrong.

The MX has proved to be utterly reliable over its almost 20 years of use!

Thanks for that, Pete, I can't be right all the time (My wife Sue says some of the time would help...).
People do go to extraordinary lenghts making adapters, but I don't think I'd bother with this one! Second hand bayonet bodies are so inexpensive it would probably be cheaper to buy one rather than a special adapter...
On a completely different subject, does anyone know what became of my all time favourite Photo magazine writer, Kevin MacDonnell?

One of the terrific things about the Pentax system is that all lenses will work on all bodies, within the limits of the body used. So, the latest lenses are quite happy on the P30 but will only work in manual focus, aperture priority and manual (also program mode on the P30T). You can even use all the bayonet lenses on the Spotmatic cameras with the K-mount adapter.
I am very pleased that I have stuck with Pentax cameras since around 1970, and have amassed a large selection of lenses of all vintages at very low prices. The quality is also superb.

I wrote that sentence the wrong way round, didn't I? Thank you for pointing it out - you are quite right, you can fit screw mount lenses to the bayonet camera bodies, but not the way round I said.

If you buy a copy of Amateur Photographer and study the prices, you should pay a bit less than the dealers are asking for a private sale. As to how much, it depends on the condition of the items.

The MZ5n is, in my opinion, an excellent camera, and you can buy it new. For me, the main advantage is that it uses a conventional shutter speed dial and of course the Pentax lenses have a traditional aperture selection ring. If you saw the difficulty some photographers get into with makes that use control wheel/menu systems you would appreciate the point. I've seen more than one camera club member totally unable to select a particular shutter speed or aperture without their instruction book!

But in the end perhaps you will have to try it and see. When you find your equipment does not do something you need it to (this may never happen of course) then, like we all did, you will move on to something that suits better. I've had so many cameras, but because of that I know what features I like and need.

In summary these would include:

Shutter speed dial
Ability to set any film speed manually
(Aperture priority and) manual settings
Aperture control on lenses
On/Off switch combined with shutter release and not a separate switch

The cameras that therefore suit me are
MX/MZ5/MZ5n/MZ3/645n/67/Spotmatics

Hope you enjoy your photography, whatever camera you decide on.

The MZ5n is available at such a low price these days, why not consider it instead? I think it's a much better camera than the ones you list.