Your price sounds rather low. I am a householder not an installer.
I've got a Vitodens 100W, seems a good boiler to me. Quiet, efficient, fairly small, well-made. I've had it since September 2008.
I chose it for the stainless internal parts and the long guarantee (provided you have it installed by a trained installer off the Viessman website list). The open-vented conventional boiler has very few internal parts, so much less to go wrong than a combi or a pressurised system boiler.
Verify that you are getting a brand-new one, not one that has been sitting in a warehouse for two years, because I understand the earliest ones had faults on the PCB. These would be fixed under guarantee, but you don't want the nuisance. You might be able to tell by the model no. and asking Viessman when it was introduced (mine is 24kW, I think the 30kW is newer).
I presume you are having this new boiler fitted on an old system. Make sure it is properly powerflushed before the new boiler is fitted, and I recommend a Magnaclean or Spirovent is fitted as well to trap residual dirt and sludge (you can never get it all out). A Magnaclean should cost about £100 plus a bit for fitting. Thorough powerflushing will take about half a day, so I wonder if it is included in your low price? While the system is drained is a good time to make sure everything else is in good condition (pump, thermostats, programmer, cylinder insulation, pipes insulation, TRVs) which your quote sounds too low to be including. You can DIY these things, but do them before the boiler is fitted. The weather is warm now so you can take the heating out of use while you do them. If draining, bale out all the mud from the F&E first and sponge it clean to avoid more filth getting washed down into the system. If you can circulate some Sentinel X400 preferably for 4 weeks before the installer comes, it will help loosen the old dirt.
Think carefully about where you want it. The flue makes steam in cold weather, and drips, so I had mine on a wall away from windows, but close to the sink so the condensate pipe could be plumbed to the internal drain to avoid an unsightly external waste pipe.
quiz your installer to satisfy yourself that the person installing it is the person who is trained, and that he has installed this particular model before. I had an unsatisfactory experience when the boss of the company I used was skilled and experienced, but his employees were not and made several errors. It did not work for about a month until after the boss came back from his holidays
