Same here, although I have the E353 (same as the E367, but without the twisty bit). The E353 has a CRC9 connector on the side which allows it to be used with an external antenna, which I have mounted outside on the wall on my house. Because I am very rural, I can't get ADSL.Can only speak from experience...
I've been with 3 Mobile Broadband since it was introduced. Early experience with the standard ZTE dongles was mixed and speed was highly volatile. Outages were frequent and sometimes prolonged. 3 C/S IMHO used to be appalling.
Glad to say that things are very different now. I've got the Huawei E367 premium dongle and I've been very impressed, so much so that I had our BT landline taken out. Speed still fluctuates and at peak times can drop to around 1 mbps but I normally get at least 4 mbps and often 7 mbps. If your Dad wants to do a lot of streaming or big downloads then PAYG MBB will be expensive. I use either 3GB/30 days/£15 or 7GB/30 days/£25 but there are other options.
3's reliability has improved no end. Their C/S is better too, though still heavily scripted. I'm still not sure I'd want to go to them with a serious technical problem, but then I haven't had one for a couple of years now. They quote their 3G coverage as the best in the UK and as we both have iPhones on 3 (after o2) I can believe it.
3 shops are everywhere and seem to stock most of their kit.
At the end of the day no spec is any good without a good signal so you need to do your home work. I'm quite close to a mast and get a full 5 bars/3G/HSPA signal.
If you google Huawei E367 unlocked you'll find a number of sources (besides eBay) that sell it SIM free and/or unlocked.
Hope this helps