I'd say it's significantly more payable than the WW1 reparations - the initial sum from Versailles seems to have been about ~600% of GDP (no really good figures seem to be available for what GDP actually was), later revised to ~350% in the early 1920s. This is talking about 300% of GDP, but biased much further into the future and critically payable in fiat money rather than gold.
This is roughly true, but 82bn of the 132bn figure of the early 1920s was never expected to be paid (baring a miracle), lowering the actual burden to around 70% of GDP, much less than you're suggesting and then the Dawes and Young plans each lowered it further (
https://eprints.lse.ac.uk/44335/1/WP163.pdf has an interesting history and useful data and you can ignore the actual research question). Even then the actual annual charges reached 3% of GDP in only the first year 1921/22 if I remember correctly, when Germany handed over lots of assets rather than having to earn gold. In practice the focus was on what Germany was able to pay annually rather than the headline total figure, and it turned out that Germany wasn't able to generate payments of 3% of GDP on a consistent basis (which was at least in part down to German politics).
In any case, assuming a 5% interest rate, payments of 3% of GDP each year and a 20 year payment period implies an initial debt of less than 40% of GDP, an infinite payment period implies an initial debt of 60% of GDP (with 3% of GDP being paid each period and 5% interest rates), and even lowering the interest rate to 3% only implies a maximum debt of 100% of GDP, even with an infinite payment period (if the interest rate is 3% and debt is 100% of GDP then paying 3% of GDP just covers the interest on the debt).
Of course, economic growth makes things more affordable, but in 1945 nobody was really expecting miracles. And even if the initial debt was higher on the assumption that the German economy would expand, this would mean exploding debt as 3% of GDP wouldn't cover the initial interest payments.
As to payment in marks, you're avoiding the problem of stopping the Germans inflating away the debt by linking it to GDP (Versailles used gold marks for the same reasons). However, this does not solve the transfer problem. To pay the British and the French 3% of GDP each year Germany has to export 3% of GDP more than it imports (in goods or services). Now these may be paid for out of taxes on Germans (so the capital account outflow balances the current account surplus) but this is still Germany running a current account surplus whether or not gold is involved, and policies that would enable this surplus weren't exactly popular in Britain and France. In the 1920s Germany paid reparations essentially by borrowing short term from the US (on private markets not from the government). Germany subsequently defaulted on these debts so this isn't likely to be an option 2nd time around (and only postpones the issue anyway). Of course if the goods never leave Germany then they may not appear in the trade data, but you suggested these transfers would be more than just the occupation costs, implying some portion ends up back in the UK & France.
In other words, payments of 3% of GDP annually may possibly be affordable but will be controversial (for the British & Germans especially) and imply both a significant degree of ongoing compulsion and a total present value of debt much lower than you suggested.
Anyway, sorry for derailing the thread, I'll leave it there and look forward to the next update, whenever it comes. Thanks again