There are various upgrades available for the Mk3 starter system. The previous mentioned Dyno daves kit and its copies, The CNW version of the prestolite motor and the do it yourself upgrade of the original starter motor itself.
The original two pole prestolite can be modified to provide this level of reliability if you have to required skill and desire the bike to still look original.
Cost of the various versions can vary enormously and it amazes me how much suppliers hike the price up for what they sell as to what they pay to buy in the base product.
Example, There is a dealer advertising on ebay for sale a Mk3 3 coil case conversion at £125. (this is a complete motor for a harley, you just use the four pole frame and brushes) Again on ebay you can buy a complete Harley starter from a dealer, brand new and made to a far better spec than the original for £84, That is delivery included. The price these dealers pay to there supplier is less than this possibly around the £40 - £50 mark.
I have modified quite a few new harley starters for Mk3 owners and i buy the base motor for the £84 price then make it fit the commando. This is what CNW do (or they get the manufacturers of the harley starters to modify them at source), The aftermarket harley starters are made in the far east or india, don't moan, as i said they are far better made than the original harley.