David wrote:

I need around 1pF input capacitance and 1 Meg Ohm, Frequency up to a
couple of hundred MHz.

These specs are probably rather optimistic, in my opinion. I would
probably consider something like a large resistor in series with the
input of a cascode amplifier (cascode chosen to reduce effective input
capacitance due to feedback). Or perhaps a resistive voltage divider,
rather than just a series resistor, in an attempt to get flat response
over frequency. But there really is no such thing as a 1 Meg resistor
at VHF. The stray capacitance across resistors limits the maximum
achievable impedance to perhaps tens of kilohms.

However, I'm not expert on such circuits. You might try posting the
question to sci.electronics.design. There are some very clever
electronic designers there (along with a number who get their jollies
from insulting others).

Steve VE3SMA