If the middle set isn't capable of running on it's own what happens if the outer unit fails?
Just for the unit, you'd need: TPWS, AWS, OTMR, CDL, GSM-R. Provision for ETCS/ERTMS would be advisable (unless you don't plan on running for too long). You'd also need to demonstrate crashworthiness to an acceptable standard (which may mean things like reinforcing corners), fit retention tanks to the toilets and have wheelchair accessible accommodation and facilities (or have a very good reason why it isn't possible). Depending on the age of the unit things like additional anti-telescoping measures (fixed couplings or the ridged pads above the buffer beam) may be necessary, and if you plan on running units fixed permanently together then some warning or protection against separation would probably be needed, but I'm not 100% on that one.
Have a look at the Brighton Belle project for an idea of how much they had to do.