YRM crossmember cost me about £165 which is more than some others advertised but it really is well made and better quality than a Bearmach one that I bought for a previous Disco.
How hard depends on how good you are with a welder and how much other rot you find when you cut the old crossmember out.
Also you need to brace the door opening to ensure it doesn't drop or warp. I welded some box section across and used two acrow props to keep it at the right height.
 

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