Both the 35i and 35d are 3.0 engines, not 3.5 litres.

I think the 35i would be fine. Better low end torque than the 4.4 you used to have, with maximum torque available from 1200 rpm IIRC. The new 8 speed transmission would be a benefit as well. If the transmission hunts on hills manually shift it to the lower gear. That isn't evidence of a problem, that is the correct technique.

I'd take the 35i over the 35d but either would work. If you want a V8, fine, but base that on your 90% usage, as towing won't require it.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue