Jay,
IRCWCC rules: With 3.5 units Lutzow can have a 1 unit pump and two stern guns (1 x 1 unit and 1 x 1.5 units). Another option is a 0.5 unit pump and 3 x 1 unit cannons. That gives plenty of firepower but is risky with such a small pump unless the waterchannelling is well planned and the pump is highly efficient.
Treaty rules: a pump output of 0.75 US gallons (not our 25% larger Imperial gallons) per minute is allowed and in cruisers can be counted as a 1/2 unit pump. That leaves 3 units for cannons which can be used as 3 x 1 unit or 2 x 1.5 unit guns. I'd go for the latter choice under the KISS principle!
Bob