Fallow are big deer, with adults often weighing over 60kg. A .243 using a heavy 100gr or 105gr bullet is sufficient but many stalkers prefer a 6.5mm or .30 calibre round, especially for bucks in the rut. I used a Blaser R8 in .308 Win with a 170gr Norma bullet.
Fallow are often active before dawn and just after dusk, so a quality optic with high levels of light transmission designed to maximise ambient light is definitely useful. I also prefer a zoom scope to a fixed power due to the flexibility it offers. You can widen the field of view for close-quarters woodland stalking or zoom in to high magnification for a longer shot on open ground.
You will also need good binos. Most of your time will be spent scanning and spotting so there is no point scrimping on quality here.