Anybody considering the old 300/2.8 for the 7DII?

I love to shoot wildlife, and occasionally some sports events at my kids' school. I have the 7DII on preorder and plan to use my 400/5.6 on it. But I've been thinking to get something faster as well. The new 400/4 is surely great, but a bit expensive. So I figured I could "build my own" 420/4 out of an old 300/2.8 (non IS) which sells in great condition for some $2000-$2200, and a TC 1.4 which I already own. The sample images of this lens look great, but I'd love to hear some thoughts and impressions on this lens, and how does it (with TC) stack up against the 400/4? It weighs 1.5lb more than the 400/4, but I don't think the weight will be that bad (I used to hand-hold a 500/4). Also, it may be tricky to get such old lens repaired if it breaks, but I figure I could just throw it away then and buy another one - twice - and it would still be less than a new 400/4.

I don't have the 300 F2.8 L but I do have the 300 F2.8 IS (Mk1) version which is simply excellent with extenders. You are quite right in noting that the EF 300 F2.8 L is no longer supported by Canon so you are entirely dependent on remaining stocks of spares with camera repairers - probably quite risky.
I have used my Canon 300 F2.8 L IS on a number of bodies without issue (though not the 7D2) but I would suggest that if you wish to use a 2 x extender then get the Canon Mk3 version as it is significantly better than the Mk2 regardless of body and lens.
Like you I am really fancying the 400 DO Mk2, but I have to eat in the meantime so it will have to wait! However I have tried out a few Mk1 400 Do lenses - they are well worth a look as they frequently go for less than a Canon 300 F2.8 L IS Mk1.
Of the 3 DO lenses I tried one was pretty mediocre and certainly not as good as a Canon 400 F5.6, the other 2 were superb. Yes the colours and contrast were slightly muted but the sharpness and resolution were VERY good. They were compared directly with my (then) Canon 600 F4 L IS with the same camera at the same range (about 25 yards). The 400 DO's actually out resolved my Canon 600 F4 despite being 200mm shorted! Yes I was surprised too.
The Mk1 400 DO has got a bad reputation but there are some excellent examples out there so they may well be worth searching for - I keep looking!

Thank you for your detailed answer! I have been shooting Canon only for a few months, so all this info is much appreciated. Good to know about the DO lenses!

I have a 7D an old 300mm f2.8 L and the 1.4 II TC, in good light that combination is really nice, the lens is really sharp on it's own and paired with the TC it does seem to drop of just a very small amount and focus is a bit more inconsistent. I use it often except when the high school stadium lights come on, then it has to come off.

That's good to know, thanks!

At one time I had the 300 f2.8 IS, 1.4xII, and 7D combo.

Together they produce great images and the AF performance is very good.