The STC1000 Temp Sensor seems to be the normal method to control old fridges at temperature required. I have not done it myself but seen many posts about using them.
I did look at the idea of using a peltier device with the idea same unit could heat or cool but when one looks at the efficiency of the peltier unit it leaves a lot to be desired. A 35 litre peltier effect on manufactures data used 3285 kWh/annum where a A+ fridge uses around 100 kWh/annum which puts me off the peltier unit even if it can both heat and cool.
I was rather disappointed with lager experiment. I brewed two Geordie lagers one with supplied yeast and one with lager yeast the former at around the 20°C the latter at around 12°C and the 12°C version was a complete flop. However the 20°C version worked A1 so why play around cooling to 12°C when the 20°C version worked A1?
Not really a lager drinker but after the experiment sorry I will in the future not bother with lager yeast and will just buy kit and follow instructions which are the same as bitter, heavy or mild.
I find around 18°C is ideal and I stop brewing June, July and August as too hot. Just a case of getting enough stock to see one through the summer. Around 200 pints in my case.