Aircraft modification such as flight control and engine retrofits etc unless they are an existing mod are going to cost too much to consider for any normal size operator. So other than removing anything you don't need, to save weight, there is little you can do on this front.
Avionics mods can save you three thing: maintenance costs, weight, and time (direct tracking and lower minimas so less holding or diversions). The equipment in an original F27 will be pretty old and hard to maintain and weighs lots. Wholesale replacement of avionics is expensive but if you can't maintain it the opportunity costs from missed flights could cover the high cost. If the aircraft has supportable avionics keep what you can and go simple. Integration costs can be very high so adding a stand alone GPS will be better value than trying to fit a fully integrated solution. .
Best bang for buck is likely to be dual TSO C145 or 146 GPS and remove the ADF(s). 145/146 allows you to use the GPS as your only destination nav aid which means you can get into more places more often. The ADF then becomes superfluous. Removing it saves you maintenance costs and weight.
So a GNS 430? Any better solutions than that? So far I found a reconditioned 430W for £4674. Would maintenance/ dircet tracking and weight save more than this cost + installation cost? I know it obviously depends on the situation but hypothetically?