The Dutch army poncho is the largest currently in serviceable conditions available real military poncho, 220 x 160 cm.
It weighs between 650 and 750g.
The original plain olive green German army poncho has the dimensions 218 × 158 cm (plus / minus 2 centimetres, depending on age and maker). Weight between 930 and 1050g, also depending on age and maker.
Lighter ones are extremely rare.
The German poncho is rubberised on both sides. The rubber also goes through the nylon fabric itself. It is extremely durable. After approximatly 35 to 40 years of service the tapes on the seams around the hood start to become loose and fall off. But one surely can glew them new or seal the seam in a different way. I didn't yet needed to try that. I never have seen a German poncho that lost its coating in regular use or in the storage.
I left one - after decades in service - for 5 years in the garden to cover something, directly in the weather, and afterwards the coating degraded. It's even a bit fire retardant. Apart from the first olive green insanely heavy US army ponchos the German one is surely the most robust military poncho ever made. It's currently sold between 20 and rather 25 € and easily available.
The Dutch ones reach the surplus market in different conditions. Some are fine, some already have a coating that falls off.
The German army issued a few US poncho liners in US woodland camouflage, but never ordered an own poncho liner.
The Dutch poncho has the grommets in different places. You can't tie the Dutch poncho liner to the German poncho properly if you don't modify it. But that could be easily done, you just need to sew 4 straps onto the liner.
The Dutch liners surely also reach the surplus market in different conditions. Some should be good, some have surely - invisible from outside - a filling with broken fibres. I assume that they use hollow fibre filling, approximately like macaroni. These can break if too old. The liner keeps the same weight but has less loft and looses a part of it's ability to retain warmth.
That's why padded clothing and sleeping bags that are used for hiking and trekking should be bought new and not used.
For boat and car camping it doesn't really matter of course if an old military winter sleeping bag serves only in summer nights in civil use.
I assume that a liner with broken filling also will dry less fast than a new one if it got really whet, because the water surely can enter the hollow fibres.
So, if you get a good one or an old one is a question of luck, and you can't find out what you have.
The Defcon 5 poncho of the Italian army is 200 x 168 cm. The widest but shortest real military poncho. A bit too short for my taste, I am 185 cm tall. But that's my standard poncho nowadays, because it weighs only 350 g !
That's only new available like the Defcon 5 poncho liner.
I own the Defcon 5 poncho, use it in conjunction with Snugpak Special Forces 1 sleeping bag and SF bivvy bag regularly as my shelter but don't own the poncho liner yet. I think about buying it but first need to buy a new large rucksack and figure out if the liner still fits in it in the end.
I am currently developing a new 4 seasons allround equipment for my personal pretty nomadic lifestyle and don't want to buy stuff that doesn't fit into my current equipment. I have to try out if the poncho liner fits additional or not.
But I am still unsure which 120 litres rucksack I will buy. Probably a pretty expensive one, so I need to think about it very well. There are a few options to consider.
D5-PO-02 - DEFCON 5 WATER PONCHO - Jackets
www.defcon5italy.com
D5-PL02 - DEFCON 5 PONCHO LINER VEST - Jackets
www.defcon5italy.com
The Italian army poncho costs in Germany new 37 €, the liner 47 €.
In your situation I would rather combine your two sleeping bags. I mainly think about the poncho liner for Mediterranean summer conditions.
There is a reason why such a thing isn't issued in European NATO armies apart from Italy. I am a bit unsure why the Dutch issue such a summer sleeping bag thingy.
We have long threads about the poncho liner question somewhere in this forum.