Old mtb to tourer conversion, need a little advice.

[Blue Hills]

I would benefit from a frame that's larger and has the appropriate mounts for panniers and mudguards and to get a solid fork in case I want to put on front panniers too and preferably steel in case I need to braze mounts on.

You don't necessarily need brazed on mounts for the fork.

Just a good solid steel fork.

Then you can fit these.

https://www.spacycles.co.uk/m2b38s215p2031/TUBUS-LM-1-Mounting-Set-T72100

 

[SafetyThird]

That's handy, thanks.

 

[SkipdiverJohn]

Looks familiar. It’s now my shopping / dump bike.
View attachment 531161

Most, if not all, of those late 80's and 90's era welded Raleigh 18-23 steel frames appear to have identical geometry and dimensions for a given size. Irrespective of what model they were sold as and in what colour, they are essentially all the same design.
The brazed frames were a bit different though, both in angles, wheelbase, and tube lengths. The welded ones are better dimensioned for tourer conversions, IMHO.