I'd agree that you're being over-ambitious.
Far better to pick one date for now, and get used to doing that (and more importantly get it right). Then when that's done (and it will take at least two or three, and more like five, years), you can start looking at other dates.
Apart from anything else, where are you going to do hospitaller shows?? There aren't that many groups that cover this dateline, and hardly any that do it well (believe me, mine does

).
And what do you mean by "national events"??
Proposed kit:
Tunic (short, long sleeved, linen, very light brown - not the right colour, but close, and leaves open for other uses)
No.
You're going for something very specific by saying hospitallers.
It's one of the very few early medieval roles that has all the kit mapped out for you.
Stick to the statute, or what's the point??
Plus, by this date, you're talking about a tunic that's knee length or longer, and front and back split, so it couldn't be used at any other date.
Trousers (linen, drawstring, very light brown - not the right colour, but close, and leaves open for other uses. Thinking of avoiding the more accurate hose, as 1: they wont be seen, and 2: this is a beginners list after all - we'll be getting a supply of whatever is decided upon for the group, and letting older hnds get their own and stop using the communal kit, and when they do that, they can get more accurate)
Again no.
You have the statute, and trousers are completely wrong, not only in terms of the statute, but also in terms of social class.
They will be seen, (front and back split, remember, through which the hose and braies do show), and they are completely, and unavoidably wrong.
If you are going to compromise on this, then what's the point in doing hospitaller??
Shoes (Simple leather. Or shoud we be looking at boots again, having a b**ger of a time finding out about footwear ?)
actually, shoes for hospitallers.
Belt (leather, but how thick, and how long? 1-2"?)
Depends on it's use. thickness largely dependant on buckle, but for over armour needs to be thicker and stronger. belts at this period for civilians would be quite long.
Padding
definitely a gambeson, but probably worn under mail of some sort (unless occasionally in the middle east - and we're not). if no mail, again, what's the point.
Helmet (simple skull cap yet again, although a kettle was suggested in the WOTR thread, and it is tempting, but means further outlay over all three periods)
Probably more likely a spangen, half or full faced, or great helm.
Red surcoat (Material?)
shirtweight linen, lined in white linen. knee length or longer, front and back split.
You've also forgotten about the monastic clothing that was issued 9they were after all monks).
A sergeant at arms would probably also have leg armour (mail, probably with either padding or arming hosen under it).
I think you have a problem here, in that the clothing is all laid out. Although you can compromise on the amount of clothes (eg only have one pair of hosen, dispense with the fur lined cloak, etc), you really can't compromise on what the clothes are.
But I think you have a much greater problem with attempting the hospitallers. How many of you are there??
How likely is it that there would be that many hospitallers all together at once, and of the same rank?
Not very, I'd suggest. If you're going to do hospitallers, you have to have people of all ranks - from the very lowest, to the full brother knights.
You'd have to have that variance of kit in order to be at all convincing, and unfortunately, this is one you can't do on the cheap.
There are also contstant disagreements about how active the hospitallers were in this country. You could portray the hospitallers as they were in contintental europe, but you might well be excluding yourself from shows run by some organisations (eg eh).
Also you're excluding women.
All in all, I'd bin the idea of attempting hospitallers - at least for the time being - and concentrate on one of the other two periods.
You can always come back to hospitallers in a few years when you all have some cash!
(BTW - don't forget that student re-enactment groups have probably the highest drop out rate of all re-enactment groups.)
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RIP Edward the avatar cat.