A lower dosage of lorazepam will mildly interact with the buprenorphine, just like any benzo would interact with any opiate. However, 0.5mg lorazepam is a pretty low dosage. Assuming you're used to/tolerant to the dosage of buprenorphine you plan on taking, you should be fine.
Without the benzo, we're talking a risk. While chances are that you won't have a seizure, taking two drugs that lower the seizure threshold is just that, a risk. I would take the benzo. If anything, you can lower the buprenorphine dosage a bit so the mix of lorazepam and buprenorphine isn't too sedating.
The half-life of Wellbutrin (bupropion) is 20 hours, which means most, but not all, of your Wellbutrin will be out of your system after two days (48 hours). I would wait two days if possible.
However, if you do take the benzo (doesn't have to be lorazepam...diazepam (Valium) and clonazepam (Klonopin) are both effective anticonvulsants as well).