What you suggest is possible in theory but very few will have asked for a HD box without the HD channels, I suspect.
HDMI picture quality on SD channels is superior to the same on a scart connection.
The £10/month fee for recording applies now if you don't subscribe to any sky package. It will be interesting to know what happens if you ask for this only on a HD box.
There are two activation signals sent to your box by Sky: one for the channels you have subscribed to, and another to turn Sky+ recording on.
There are a number of differences between a Sky+ and a HD box, but essentially they function identically. HD has larger hard drive.
You will always get BBC HD on a HD box even if you have no subscription to Sky.
BBC HD is a superior offering to SkyHD, in that all of the content is in HD. On Sky, content on a HD channel will be in SD part of the time, particularly on Sports. The only channels that are HD all of the time on Sky are movies HD1 & 2. My view is that the production equipment and cameras are better on the BBC channel, have a look at Planet Earth and How we built Britain in HD.
Sensitivity is another issue, all Sky receivers are very close in performance these days in fringe areas. The only HD box at the moment is made by Thomson (Pace bring one out at the end of the year).
Both Thomson (SD & HD) boxes are a slightly worse than Pace, but the difference is very small. The best Sky+ box is the Pace TDS 470n Sky+ Version 3. The tuner is identical to the old PAce 430n, but quality control is better than the 430n, you know it will work well.
The very best box for fringe remains the Pace 2600 CI, all you have read about it is true.
However, digital being an all or nothing affair, even a Pace 2600 will not save your viewing if the signal is declining in the evenings, as it does in Madrid.
If, for example, you are watching BBC2 in Madrid around 10 pm on a 1.8m in the west of Madrid, the Pace Sky+ V3 picture will start to break up.
At about 10.20 you'll notice some disturbance on a 2600. By 11-00pm, the Sky+ picture will be unwatchable, but you will just about be able to get sense out of a programme on a 2600, with regular blocking.
Note that any recording made on a Sky + will play back with any break- up removed, making it impossible to watch.
A Pace 810XE HD FTA receiver is a good alternative to a SkyHD. You can get these for under £100 in the UK. Picture and sound quality (Dolby 5.1 on BBC HD) is very high, all SD looks better, but of course it is a FTA box, not suitable for everyone.. Sensitivity is the same as the Pace Sky+.