Furman P-6900 AR E on the cheap, should I go for it ?

Yup, 30A feed (but I'll have to install another wall socket, right now the schucko is specd for 16A).

Voltage going over the 240 nominal value. steady at 248V, not good for vintage gear in the long run. Some expect 220, even 210 ?

Just a FYI - it won't protect against a lightning strike. (Personal experience there). I haven't noticed any noise reduction. They do tend to smooth out power delivery to 117v - US here. It might offer some peace of mind. Not sure if I would buy them again. My power here is pretty stable......except for lightening.

So - what SHOULD we buy that offers some protection against brown outs?

Your asking the billion dollar question, but first we have to ask the more basic question, how do I know I have an issue? Sure you could watch the voltage display on your typical over priced and hyped rack mount strip but your going to only be there for a limited amount of time so the sample size is way to small. Even then is that VM in your rack quik enough to show it? A quick logging Voltage meter could tell you there is a problem but only if it was plugged in when those connections exist. An example from my old road warrior days; there is a banquet hall in Racine WI that would give you sporadic spikes. These spikes only occurred if the dish washer was running and at the exact same moment the HVAC kicked in and the compressor on the walk in cooler kicked in. You fine unless all 3 presented at the exact same time! It took 4 days of experimenting and investigation to figure this out.
Probably should have just rented a Showpower Kohler generator ( did I mention on the night that lead to this investigation 2 VPs from Kohler were in the audience).

Now for the irony of this all, years ago Mr Furman told me “ you know if manufacturers would build gear with really good power supplies I wouldn’t have a buisness”. That’s not going to happen.

I personally stick with Furman as long as the unit has SMP, so that even after it has protected me from a spike, it still keeps protecting ( kind of Duh don’t you think). Some of the very high dollar pure sine wave regeneration units probably do a decent job, but you’re talking big bucks and limited current. Some swear by Balanced Power Transformer based units, but they don’t really protect and honestly I’m not sure considering every PSU in a piece of audio equipment I have ever seen uses PSU’s designed assuming they will see single ended power.