If your only having a few lights you wont need to uprate the alternator, its unlikely roof lights will be on for any great period of time so what you have should be fine as long as the battery is in good condition.
I run a CB, PDA, Stereo, and a dash board gizmo without any problems and I have a lower rated battery than I should have, but it was free

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Save your money to spend else where would be my advise.
