Chris, Welcome to the wonderful world of the Mamiya RZ. Now that you have chosen the 140mm lens, you will have to pay a bit more for that beauty of a lens. This may financially limit your choice of a standard lens; 90mm, 110mm, or 127mm.
If you think you can't afford the 110mm, not to worry. The 90mm can be had very economically these days, about 95 GBP, or 115 Euro.
You may find the slightly wider 90mm able to render landscapes with a more preferable perspective than the 110mm.
That's a personal call though.
The 127mm, for now, would be to close of a focal length to the 140mm, (not enough variation/difference).
The 100-200mm Zoom is an acceptable lens, but of all the lenses in the RZ line-up, it is less than stellar. When I only had one other lens for my RZ, the Zoom filled a gap and was useful at that time. In hindsight, I think I might choose to skip that lens, unless you can get the Zoom for less than 300 GBP, with the support bracket.
There are three focal lengths, (five lenses) that require a support bracket in the RZ line: The Zoom, (there is only one version), the f:6-360mm, the newer f:5.6 APO-350mm, the older f:8-500mm, and the newer f:6-500mm. Please do not use these aforementioned lenses without each of their accompanying support brackets. To do so places severe strain on the camera body's rack focusing rails. There are (3) separate brackets: the Zoom, the 350-360, and the 500's.
I actually used my 350mm without the bracket, but I supported the lens and camera on a pillow, on top of a board, (small bread-board size piece of plywood), fastened to the top of a tripod. Sort of like a sheet-music stand, but turned into a flat/level table, not tilted. This is how I still, (on rare occasions) use my Zoom with extension tubes, and sometimes my 350mm with tele-converter.
Pillows, or more appropriately, bean bags and/or sand bags, can go a long way in supporting any camera, and especially the heavy RZ, for almost no money at all.
Take the lower legs of a pair of worn out children's jeans, or sleeves from a wind- breaker jacket. Sew or tie one end shut, fill the pants leg/sleeve full of dried beans or rice, then sew or tie the other end shut. Rice can be a little dusty; just put the volume of product necessary inside a zip-lock baggie first, then into the pants legs. I prefer the rice, but either works well. Navy beans better then Kidney beans.
Don't get too anxious about buying 220 size film backs for the RZ. The available variety of emulsions in 220 continues to dwindle. You will find far more use in having two or three 120 backs, before even considering a 220 back. Your shooting style, and film availability may vary.
If you want a Polaroid back, do not buy any backs that are marked 545 or 545i. These are not for Polaroid "Instant" photography of today. These backs are for single-sheet, "conventional film" sleeves, of which few are made today. These are the backs you will see having a large silver toggle lever on them.
***The 545/545i backs are totally useless for "instant" photography today!***
For instant photography, you will want a film-pack style of back (most will be Polaroid branded), but these will accept Fuji FP100**, and FP3000** series film, (the FP100** "45" series is for larger 4X5 view camera backs). There are ten (10) sheets a pack, which is still being manufactured by Fuji, and is widely available. Good film, and only about $10-$12 a pack on my side of the pond. About a buck a shot, (one dollar), not bad at all. Less than a quid per shot for you.
The larger 4X5 backs were Polaroid 550 backs. The smaller medium format size Polaroid backs (for your RZ) were 405 backs.
Even though the film is 3.25 X 4.25 inches, you will only expose a spot equal to your camera's format size, which is 6 X 7cm, with formatting masks, or about 72 X 72mm square, without masks. The masks are like a dark slide with either a horizontal 6 X 7cm, or vertical 6 X 7cm hole cut out, as the Polaroid back does not rotate. The masks are seldom included with these backs, but make sure it comes with a functional dark slide.
The link below shows how to load a pack film holder with instant film. Too many people try to over complicate this first step, only to ruin many exposures. In this video, they are loading a 4X5 pack film holder for use on a view camera, not the 3.25 X 4.25 inch film holders available for medium format cameras, but the loading is the same. Don't worry about trying to feed film tabs through the rollers or any thing like that. The process is very simple. Put the film pack into the back, close and lock it, then pull the black tab. The first white tab is automatically drawn through the rollers, ready for your first shot. The Fuji film pack is just slightly larger then the Polaroid packs, but with just a little extra force the Fuji pack fits, and delivers excellent results in a Polaroid back.
Oh, make sure your dark slide is in place.
Read the temperature and time chart to determine how long to wait before you peel apart the film.
http://www.youtube.com/watch?v=Yd4yREiHogg
Here is another, this time loading a pack film holder for medium format
http://www.youtube.com/watch?v=ComB9GmNlKg
And another, this time on a RB
http://www.youtube.com/watch?v=dAC2tEQEgFI&NR=1