Overheating problem - head gasket replacement or sealant?

Hello all, I have a 3-litre diesel Grand Cabin campervan, '97 vintage.  It overheated recently - went into the red and cut out.  The garage flushed out the coolant, which was pretty mucky and looked a bit frothy to me and checked the thermostat.  I have done a couple of short (half-hour) trips since with no problem.  However, the garage said that although their testing gadget detected no exhaust gasses in the coolant the first time they tried, it did detect a small amount the second time and they are proposing I replace the cylinder head gasket and recondition or replace the radiators.  They are reluctant to name a figure, but suggested £1.5k - 2k when pressed.  Noting my glum expression, they suggested I try using K-Seal, or similar, though they warned that this could end up clogging the radiators and making things worse.

Does anyone have any experience with this kind of product, or other suggestions?

Oh dear, sorry to hear that. Is it the 1kz-te?
Bear in mind the stock temperature gauge for that engine hides (damps)  the vital activity from you - I've done a thread about how to modify this so it keeps you up to date on engine head temperature.

First things first: the cause of the overheat. You must fix this first IMO.
The most common cause is a faulty viscous fan, they are cheap on eBay and that should be your first fix. You may also need a new thermostat: get one fitted anyway as it will need it one day.

The next suspect is the radiators - if it still gets too hot get a new main radiator - or get it to a specialist.
Same for the 2nd radiator (assuming you have one - the 1lz-te models do).
Also test the electric fan works but switching the ignition on and unclipping the wire directly below the 'radiator' cap - the sensor on the base of that elbow if the fan temperature switch. Do you recall hearing the electric fan on when it was overheating?

Do not put anything in the coolant except for Toyota long life antifreeze and deionised water. No fixes or additives. K-seal etc will insulate the coolant from the metal and inhibit cooling, which is 'just ok' on these vans anyway.

Only once you are sure you have fixed the cause of the overheat would I consider a fix with a new head gasket. A gasket would not be that much, but a new head would and it sounds like you'd need to shop around - if you pay real money you'll want the head crack testing and a new Toyota head to fix it properly if it is cracked. The 1kz is a strong engine but the head is prone to cracks if overheated.

Thanks, GPW. I believe it is a 1 kz-te, based on age and cylinder capacity. I'll check the fan and thermostat and then see about getting the radiators overhauled or replaced.

BTW you should be able to hear the main (viscous) fan roar when it's working, rather than just spinning - it should be moving quite a bit of air when it's just warmed up as it's a big fan.