the iron rad for the same size new rad will be about the same as the panel withought the convector attached-the new one will give a little more btu as its thinner metal.
1000x 400 is quite small for any room as 1000x 600 is the norm. However, you will have 2 radiators in the K+ giving you 400x2000 effective panel. with the addition of the convector it will also provide more heat output.
A 400 x 600 with one convector fin will provide an overall panel approx equivelance of 400x1400 compared to your old rads.
the right way to do it of course is to re-calculate your BTU rating for the house & room and apps available online easy enough to use then use the requirement to cross check what rad to purchase.
Taking all that into concideration and the fact you will need to adjust pipe feeds anyway, why not go for 400x800 k2's with trv's?. last thing you will want is under-performing heat output!
http://www.simplifydiy.com/plumbing-and ... le%20panel
its been a long day