It's not unusual for an acquirer to pay for a business over an agreed time period. It would be fairly unusual, however, to have equal monthly payments. If there are no contingencies (such as an earn-out period or revenue targets), it would be more common to get over 50% up-front.
I'd check the acquirer's solvency & cash balances. For e.g. if it was Microsoft, I wouldn't lose too much sleep over whether you'll get your cash.
If there are any concerns over solvency, then have them enter into an escrow agreement. Or have a third party guarantee their payments (at their cost). An escrow agreement would be more common, but I'd imagine that the desire to spread the payments equally over a 24-month period relates to their cashflow position.
I don't think you should sell the debt for less than the €400k- if that's the acquirer's offer, then really they should be good for it, and be able to prove they're good for it.
Another option is to make the transaction reversible if they don't make payments, although this may make it unattractive to them.
You should seek legal advice from a commercial solicitor before you sign a term sheet - that's the place and this is the time where professional advice is most valuable. You should probably take some tax advice to figure out how to get the €400k out of the company (if it's an asset sale, the money will go into the company). If you own the assets in your own name, I know that you'd be taxed for CGT on the whole amount up-front, even though you don't actually receive the cash. There are pitfalls that are fairly easy to avoid if you have the right preparation and really the only place to get that is from a professional advisor.
Best of luck with it.
Sprite