Best rails position on frame in a dusty workspace

Keep in mind that for accelerations of 3-5K the moving parts have to be very lite and very rigid, any slop and it will end up visible in cuts.

Yes I know, I will try to do as lite as I can.
What do you think could be a good weight target for a 1,5m long gantry?

10-15KG

I would have said stay < 30kg with gantry ends, Z axis and torch, wires etc.
The torch lifter is hard to get under 5kg, the torch is about 1kg so Tommy's target implies use of belts which you may or may not want to use.

Thanks for those details Rod.
What is your opinion about belts?

I have seen that Tommy use it on his tables, but I mainly see rack and pinions on commercial tables, and on DIY ones too.

I used to use belts on mine (HTD5M), I now use rack and pinion MOD1, bought here if you are in europe (as they were very cheap imo)
www.motedis.co.uk/shop/index.php
they ship from germany
they also have a lot of other motedis websites in US, mexico etc etc.
they are not into chit chat, just order and send whatever you order there is no phone support and the email support is "interesting" if they get something wrong (I asked for 8mm pinions and got 1/4" pinions....)
www.motedis.co.uk/shop/DIY.php
but I would buy from them again anytime

I know a guy that uses chain..... yes chain and sprockets and he has made I believe 9 tables and sold a lot of designs for others