The point of a shiftkit as I see it isn't so much to make the bike "faster" - no more so than say swapping out the stock 44t for a 36 or w/e, it's to use the engine's effective RPM range more efficiently, which in turn increases acceleration and helps overall reliability and comfort by reducing RPMs at cruise speeds.
For this, I think three speeds more than adequate. Remember many automatic transmissions in cars are three speeds, it wasn't uncommon in some older standards either. I've said to more than one friend that while I love my Schwinn (no shiftkit) it's a heck of a lot like being stuck in 3rd in a 5 speed car - a lil boggy to get going, then it winds out too quickly, maxing out and screaming as I cruise at 30mph.
I will say that some 3 speed hubs just can't take it (older used ones in partic), those that can may be cost prohibitive - yet this is kinda a moot point as is even the three speed question as should the hub fail and/or you decide you need more gears - it's simple enough to add a rear derailleur and rear wheel w/sprockets, should it have come frm a 21 speed - now you'll have a seven speed shiftkit etc.
As for the freewheel bearing - yeah ya defo outa get the HD freewheel from the get-go, it's kinda a pricey item and it comprises much of the cost of the kit. I suspect the only reason they offer the lower quality "standard" kits w/o the HD bearing is because so many have lamented the cost of the kit in the first place... which, no offense to anyone - I think it's just crazy to try and compare the cost of a mass-produced in some hovel in China engine with a quality, custom specialty item like the SBP shiftkit *shrug*
As for "more of a problem to start" - I've heard this before and don't really understand it... sure, ya can't just drop the clutch while yer rolling, you actually hafta pedal. Why this is a problem I dunno - my Schwinn (no shiftkit) can't start w/o pedaling either as the compression is to great and it'll happily skid to a stop lol
There are some potential problems with getting a shiftkit - frame clearance and install can be a touch tricky, the kit's chains do need to be watched for proper tension, and while the clutch cover's fasteners can be reversed (I heartily recommend that mod too) to allow the cover to be removed w/o disassembling the shiftkit, should you need to service the clutch actuator arm or the engine's sprocket you'll need to remove that sideplate to get at it - not a big deal really, the clutch side is the complicated one (thus the reverse the fastener mod).