I too tend to agree, buying the most original one you can is ideal, though not always possible. Colour changes are regrettably a little too common, and I too have been guilty of this. But often such matters are dictated by personal likes and dislikes.
Resale Red being the most obvious and common colour change from other colours. I actually really like the Giallo Ochre colour, as it is one which really suits the shape of the 105 GTV.
My GTV is white and I am not a fan of white. So given that I have had this car now for 13 years plus, it is tired on both the exterior and interior, I am afraid that when I get around to restoring mine, I am planning on changing its colour but to what I don't know, but definitely not red. But you never know, perhaps not !!
My Giulia Super has had an external colour change (from a dull burgundy colour to blue) as it was totally rusty and needed new doors, cills etc, but it is not a car I intend selling and I am building it up to suit me. However I kept the interior the same colour, except for changing it from vinyl to leather.
If I were on the market for either a 1750 or 2L these days, as I mentioned previously the first thing I would look at is whether or not there was rust and/or signs of any crash damage, if only minor, then I would consider the state of the hard to find/expensive trims (ie stainless steel parts etc) and lastly the mechanicals. The mechanicals on 105s are robust, but obviously do wear with time. They can be a little expensive, so look out for problems with the gearbox. The common problems being loss of synchro on second gear, (although other gears can loose theirs), also reverse gear lock out failure. That is reverse not holding when you need to use it.
In terms of the engine, the obvious signs such as excessive smoke when starting, accelerating or when de-accelerating (are symptomatic of either rings and bearings or valve stem seals/guides or/burnt valves).
Problems such as missing, running on 3 cylinders can be due to things such as poor timing, poor tuning of the carbies, or possibly head gasket failure. So obviously check the oil and water for any signs of problems. Noisy/clattering sounds in the engine may mean wear of the double timing chains, or worse ie. piston slap. Ouch that means opening up the engine.
The other thing to look for is the state of the suspension both front and rear. If tired you will get all sorts of knocks, squeaks, or even wonky steering/handling. This usually means going right through the suspension, new shocks, bushes, tierods, bearings etc. Again this all adds up in terms of $$$.
Hence the consistent advice of trying to get the best example for your hard earned $.
Test drive any/all you can is often the best option to finding one which suits.