Hay Capt: You may wish to try Gibbs, as they sell the SMLEs after they have reworked them. Their rifles look great, nicely finished with new stocks and I understand new parts where necessary. But, I think they are mostly in the form or length of the Jungle Carbine, which would make the barrel a bit shorter than you would like it to be.
I think that if you want a MKIV with a 24 inch barrel you will need to find a good one and then have the barrel cut and crowned. They MKIV is the only one that comes with a military peep sight. The No I MK III had a 'v' notch ladder rear sight. Check your shotgun news - I see the SMLEs in there a lot. Also, you may wish to contact Gibbs and see what they can/will do for you.
Hope this helps. Mikey. Merry Christmas.