You might want to look into what the NRO is already putting in space (supposedly) [1]. If the reports are to be believed, 100m dishes are already there, just pointing at the earth instead of into deep space. To your second point, it seems that relative positioning is mostly a solved problem at least for satellites in GSO (that's a core part of how GPS works). My personal opinion is that space-based radio astronomy is mostly a problem of cost, not of available tech.