Congrats on the purchase, I've had my Shen for a bit over 2 years now and am well pleased with it. Sounds like you have already found
www.largeformatphotography.info, but if not, go there - everything known under the sun about LF!
For lenses, a rough rule is that the focal length equivalent is 3x a 35mm lens and 2x a 6x6mm lens. So if you find you favour a 28mm lens on your 35mm camera then a 90mm lens on a 4x5 gives about the same angle of view - it's not the same as the different film aspect ratios confuse things. Likewise, an 80mm lens on a 6x6 camera is about the same as a 150mm lens on a 4x5 camera. As already said, the Linhof style lensboards that fit the Shen are widely available. You need the hole in the board to be the correct size for the lens's shutter so wait until you know what lens you want before buying a board for it.
You need some method to develop the film... I've always used a Jobo CPE2 and 25xx series reels so can't help much there (unless you have the same). Dish/tray processing is a black art I have never attempted to learn.
Focusing cloth, more film holders, loupe, shutter release cable, more film holders, pocket spirit level (the bubble level in the Shen's back is a nice decorative touch, but...), ground glass protector, more film holders, Polaroid holder (Type-55 is wonderful stuff), lens wrench... did I mention film holders?... Frankly, the list goes on forever

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Have fun! Bob.