If you have patience, tools and the time & space I'd say give it a go yourself, or at least do as much as you can - leave any machining work to specialists but do the strip down and assembly yourself if you feel confident? These aren't hugely complex engines but you do need to be scrupulously clean and methodical.
If you're after some guide prices, you could ask Tim Williams (on Facebook), Shem (
https://www.facebook.com/shemtekautomotive) or there is an indy in York who is supposed to be very good -
https://www.dyrdalsgarageyork.co.uk You could ask any/all of these places for a quote - that would give you a ballpark figure to consider.
The other cheaper alternative, albeit not ideal, is to simply drop in another engine. Get an early S70/V70 B5234T3 engine as this has slightly stronger pistons etc. as shown in the attached technical bulletin. It is otherwise mechanically identical (barring a 16T turbo if its come from a manual car) and you can re-use all your sensors from your existing engine so it will continue to run fine on your Motronic 4.3 ECU. Obviously you'd need to make sure this engine is better than the one you have
NB I realise you are in the Netherlands (?) but I can't recommend anyone over there as I have no knowledge of them, hence me suggesting a few that I do know of - and would use myself.
HTH