I am purchasing a new plotter (HP T830). The dealer tried to sell me a $200.00 line conditioner. Is this really necessary? In the past I have always used a $50 surge protector. Any recommendations on what I actually need would be most appreciated.
If I was spending $6K on a new plotter, I'm not sure I would skimp on power conditioning. I'm not sure what the dealer is trying to sell you, but I've spent nearly that much (your $200 quote) on a power conditioner to protect my audio components. I might shop around a bit to compare what the dealer is trying to sell you and measure it against what you might get elsewhere for similar money or less. I use a Panamax unit in my main home-audio stack and am quite pleased with it.
Any quality midrange or better UPS should work fine to protect you from both surge and undervoltage and other types of major power issues. I use APC and Cyberpower usually between $50-150 range depending on the load. For just the plotter a 600VA unit or better should work fine. I don't think actual power conditioning (noise reduction, interference, etc) would be a huge factor for a plotter but might want to check the plotters docs just in case, but I highly doubt it.
There are surge protectors, and there are battery backup power units. $200 should be buying you a pretty powerful (dual battery) battery backup unit. Battery backup have a inverter so they have high degree of isolation from most power line noise, surges and brown / blackouts.
I would use a good battery backup on your computer, and a good surge protector on your plotter.