What is the Y plus value i should use for running an aerofoil simulation at very high Reynolds number?

it all depends on what turbulence model you use and whether you use wall functions or not. It will generally have to be either less than 5, or even less than 1 (e.g. S-A) or between 30 and 100 (when using wall function, say with k-e). NB Always try to avoid the buffer layer (5<y+<30).

Why is buffer layer avoided?

In generall, turbulence models are designed to work either in the logarithmic layer (where empirically based wall functions can be used to estimate the boundary layer profile near the wall)or in the viscous sublayer (where a fine mesh is used to resolve the layer) but not in the in-between buffer layer, where the effects of molecular viscosity and turbulence are of equal importance.

for more info look into your solver's manual under near wall modelling.