My unit is a Hach DRU 4000 with a pour through module. It is 15+ years old.
The unit uses two bulbs. A tungsten bulb for VIS and Dueterium bulb for UV. The bulbs have a shelf life and it is taken up in warm up and runnning both. So turning on a bulb from cold puts about as much wear on the bulb as if you left it on for 8 hours or more. The unit is fairly slow at producing a spectrum scan but obviously you are wishing a single wavelength scan. The pour through module makes this pretty simple because you do not have to remove cuvettes.
I would not recommend a used Hach unit. The one I have works for the data but the old ones are notoriously finicky. There is a multitude of seperate circuit boards inside as was the design style back then . Power supply boards and the interconnects between boards are with plastic electrical clips on the wire ends that plug the boards together. I worked a career as a control systems design engineer and saw this over and over because I had to design with this kind of component. Over time those connections tend to oxidize and resistance builds. The problem with an oxidized connection is that it is not like either a broken wire or a short circuit and so a voltmeter is very tricky to use to track down problems. Voltages must be read across the connection which most electricians will not understand to do to track this stuff down.
All sorts of glitches creep with oxidized connections and components that will drive you nuts. Error codes will come and go and you have to know where to bang the cabinet and why to make some things boot up lolz. I am certain without an extensive electronics and motion control machine design knowledge a typical user would have returned the unit or trashed it immediately. It has value to me because I have only used it to take single measurements here and there and it is hobby. When I hit the lottery again, or at least if I make it through my disability review, I am looking at a new uv/vis unit.
The new models are surface mount circuitry and generally like all electronics are much more reliable. Plus a unit with a halogen bulb likely never needs the bulb replaced unlike standard tungsten and Dueterium bulbs. Old units like mine needed several minutes to warm the bulb up to temp. Halogen bulbs provide both UV and VIS light and need no warm up. If I sacrifice a small amount of resolution and select a model that scans the entire spectrum at the same time then every scan will have the entire spectrum stored in the same time it took me to scan just one wavelength on my old machine. This type of unit is called a photo detector array unit and they sacrifice a bit of resolution for the incredible option of full spectrum scanning instantaneously.