Hi, go to
the second most helpful website ever after this one, and under maps - southern berg you can download the old versions of the maps. From what I hear they are predominantly the same as the new ones, other than some funnies (eg Mlambonja is 3306m instead of 3256m).
Otherwise you can probably get them from Trappers or Cape Union Mart (I'm sure they could order them for you). Or I'm sure if you arange it far enough in advance the guys at Sani Top could get the maps for you and you buy the maps off them (assuming they don't sell them as a standard). Email them, their details are on their website.
As to how long it should take, 3 days would be a hang of a push. I assume you are descending Iscutulla Pass to get to the small berg escarpment at Bushman's Nek? Even 4 days should be a push.
I don't know Cobham and Bushman's Nek from experience, only from maps and photos, but bear in mind that Garden Castle area has many significant ridges, if you don't stick to the escarpment edge you should be able to cut across and avoid the Hodgeson's (which would be such a shame to ignore Hodgeson's South Peak), No Mans Peak etc. However, once you reach Rhino (the escarpment is around 2950m here) you need to cross the Matebeng ridge, the Mashai/Tsepend ridge, the Mzimude ridge and the Walker's ridge, not to mention the descent down Iscatulla Pass. Its easy on paper, but in reality you have to gain and lose around 300m in altitude for each ridge (bearing in mind that Mashai and Walker's Peak are both in the top 25 highest points in SA, not to mention Tsepeng being the 8th highest point in Southern Africa).
IMHO its best to take longer over these routes (if you have the time, or alternatively reduce the distance), it allows you to summit (at a minimum) the most scenic peaks, in this case (by order of reputation, but only including really awesome peaks):
1) Hodgeson's South (which is far out of the way if you are hiking to a tight deadline)
2) Rhino (which adds 4km to the hike)
3) Walker
4) Wilson
5) Verkyker
6) Andre's Knob (maybe not the wisest peak to boast about climbin

)
Not to mention the other amazing peaks in the area...