Hi there, I don't think the thermometer is inaccurate. The temp you got is quite normal for a tank with a few pumps running.
As for chiller, I think it's a great investment. It'll maintain the temp of the water, less evaporation compared to fan.
Minimally you should go for is the resun 280. Resun 150 is way underpowered in my opinion, it may just keeps running.
I was previously using the resun 280 for my 2ft cube. It's a very quiet chiller(the 280), but was slightly underpowered for my 2ft cube. So i guess it should serve ur 1.5ft cube well. Look out for it at pasar malam, pricing is very affordable.