Awediohead--As I recall the measurements are outside/inside diameters (there are different sizes, I presume), and the 'HP' compatible types all seemed to work; I actually ordered several different ones 90/120 & the beasty one.
Sorry for non-response--haven't been on here for a while. I think with your proposed (lo-power) setup, a 90W is pretty marginal--don't do it. However, 120 should be adequate; the price differential between 90-120W is negligible, and 120 would allow for more margin and the possibility of using upgrades in future.
. Since it is external, there is not an issue with heat, per se, but it seems that a higher watt unit would not be putting off as much if it is rated for higher wattage than you will be actually pulling, yet the internals will be thicker gauge or more sturdy, as a whole, have more resilience or heat tolerance and hence last longer.
Although I originally intended this unit to reside in a LG AIO 'kit' , mine is now in a 'drawer' computer setup, and I have a lot of externals & internals plus fans and USBs constantly hooked to it, and I have not throttled it whatsoever, so I have the capacity to have it ramp up and use more watts, as needed. OK; the 230 might be like hitting flies with a hammer, but it wasn't that much more $$ than the lo-watters...
Frankly, I think that I rarely even evoke an increased cpu fan speed--can't remember even hearing it (lol). However, I use it pretty much 24/7 and do video renders, and typically 5-6 apps open at once. I always have the option to ramp up my cpu and case fans and use the full power of the I5 without ever worrying about heat or power with my 230W. (I have no idea where my other PSs went, but this 230 is HP branded OEM unit--a BIG 'brick'). In sum, I would personally recommend you use a good-branded unit with proven use/reliability, exceeding nominal ratings for any perceived upgrades or use. Hope that helps