Ok, to give the Saiga normal handguards, you need to replace the gas tube and get a handguard retainer. To put a real AK handguard retainer on, you need to remove both the Gas Block and Front Sight Block. Now, there are two methods that I know of to avoid doing this. Dinzag sells a bolt-on retainer so you don't need to remove the GB or FSB, but they are expensive. The other option is to take a dremel and drill to a regular AK retainer and convert it to a bolt-on.
As far as which Saigas will accept High Cap mags, as far as I know, the 5.45 and 5.56 will accept high caps right out of the box. The 7.62 will need to have the mag latch filed down some. The 7.62 also needs a bullet guide to feed properly. I have heard that the 5.45 and 5.56 don't really need it, but I would add one anyways. They aren't hard to install at all.
I am pretty sure that the majority of Saigas being imported now have the barrel cuts. If you get an old one, they may not. Another thing to look at for when converting is if it is cut for a pistol grip nut.
A dremel and some hand tools will convert a Saiga. Even if you don't have a dremel, a file will work as well. The work isn't hard to do at all. I did my 7.62 conversion in about an hour, if that.
You also have to make sure you are 922r compliant. So you need to make sure you add enough US made parts to the rifle. Usually, a new trigger group, furniture and gas piston take care of that.