Hi Keith. Welcome to the forum and to a great little car. Sorry for length...
I would concentrate on re-building the original transmission as well. It keeps the car original, and all the tough 'custom' stuff is already figured out for you, such as shifter placement, exhaust routing, driveshaft length, transmission mount, etc. I believe this will end up the most economical option for you.
Alex Black had a very detailed thread regarding putting the Lotus five speed in his Elan. I believe he had some advice in there somewhere on ensuring the shift quality is as good as it can be. Take a look or PM Alex for a few details. The key point is this thread provides an overview of some of the hassles encountered in swapping a box, which would get spendy quickly if you are getting the work done.
drive-train-gearbox-final-drive-f37/gearbox-swop-t21414.htmlI can't speak to the Voight box. The MT75 might be able to be fitted in a 2001 Spyder, but pretty sure some modification will be required. If you give Andy or Sean a call at Spyder with your Spyder chassis number handy they can advise you on what you have. The 'later' Spyder chassis I have (purchased around 2008) was modified to make it easier to install the Zetec and MT75; I am not sure of the change-over dates and exact differences but Spyder will know. At a minimum I bet you will need to modify for the starter. Believe the other mod they added is two tapped positions for the transmission mount; take a look at this area on yours and see if you have extra tapped holes there. Not sure of Lotus 5 speed mount details, but sure there are photo's in Alex thread above.
Even with this frame and a Spyder supplied MT75 and all parts to connect to the Lotus TC this was not without adventure. A bar in the chassis web had to be cut out and modified to accommodate the starter, I had difficulty with the pilot bearing and replaced with a bush, the reverse light switch was difficult (new part is NLA), the shifter position moved slightly from the four speed requiring custom interior mods, I had to fabricate proper sheet metal pieces to cover the clutch housing, the exhaust needed re-routing and still is very low around the bell housing, the clutch is completely different, etc... All of this was done during a frame off, which makes some of the preliminary fitting issues easier. I shutter to think of the cost of all this if I was having all the work done.
Still very happy with the MT75 conversion, but we didn't really get the 130/5 in Canada in any kind of quantity, so it was the only practical way for me to get a five speed. If I had one already, I would just get it fixed.
One thing I would definitely try to do is drive the car a bit to get an assessment of any other engine work you may want done while it is on the bank. Even little things like an oil pan or cam cover leak are easier with it out.
Not sure of available services in your area, but for sure they are way better than my area. Perhaps you can keep costs down by having a more general repair shop perform the drivetrain removal and installation and send the box to a specialist for proper rebuild? Just a thought.
HTH