Whichever bike you buy, if you can afford it then get a new one. Just because older models were better doesn't mean the previous owner maintained it better. Remember, you will have to spend more time and put in more effort to maintain an old bike, maybe more money as well. If you like doing that then its OK, else go for a new one.
The quality of engine is the same between old and new models of GS150, my bike (2008) has never given me any engine trouble. I have never had any problem with the wiring as well.
The chrome work is not good on the newer models, you will have to take extra care for the exhaust pipe from the head to the silencer. The speedometer drive assembly in the front hub has known design fault, one has to take extra care and clean the speedometer cable periodically (3000-4000 km).
The consumption depends on the way you ride, it normally gives me ~45km/l and the last time I came to Abbottabad from Lahore I rode at 70-75km/h and did not exceed 80km/h (6,500 rpm), it gave me 52-53km/l.