At the onset, let me thank you for addressing us as experts, however, as a mater of fact, we are ordinary car buyers like you'll, who just happen to love cars, therefore this auto portal. However, please do take your own decision which need not necessarily be based on our suggestion. Treat our inputs as suggestion only.
Yes Sunny is a smart decision, just on space, its far to comfortable than the Dzire, which is a sub 4 mtr car. Sunny therefore offers more legroom, shoulder room and cabin space than the Dzire. With 490 Ltrs of Boot space against 310 Ltrs of dzire, even your luggage travels in comfort
With more advanced braking safety features like ABS, EBD and Driver side airbags, its safer too.
Lower Kerb weight than the Dzire, with an almost lag free diesel engine, the drive should also be good.
Yes your concern on service cost could prove to be true in the long run, even though we do not have much information on it as of now, but as compared to Dzire, service should be expensive just because of economy of scale which is hugely in favour of the Dzire. Other reason service cost could be high would be dealer cartelisation as there are fewer dealers for Nissan today.
I have taken the liberty of pasting 2 links, if you wish to study these cars in details
Nissan Sunny currently runs on 14 * 5.5 J Steel, 185/70 R14 Tyres. whilst it is always advisable to go with OEM fittings and if one does want to change then keeping the dimensions same, one can opt for Alloy instead of steel rim. Under normal circumstances, using an upsized Alloy by around 1 inch extra should not be too much of a problem, however, if you were to do so, then even your car tyres will have to change to suit the 15" Alloy. A decent Alloy wheel should put you behind by around 4000 to 6000/wheel. However, beyond aesthetics, one does not get too much benefit from alloy wheels.