
I'm Rob a relative newby on here i'm wondering if anyone could offer any advice regarding what i think is knackered head gasket

I bought my van in november last year its a 1988 caravelle 1.6TD JX the kilometerage is 302,000 kilometeres, i was checking the coolant levels the other day and noticed that there is a very dark substance in there(the tank thats not behind the flap), also in the cap there was an almost black slime/sludge, is this definatley oil? I read on the wiki on here that if coolants are mixed then sludge or slime can occur. The van has a leak on the cooling system on the rear heater. Maybe theres a whole load of different coolants gone in it over the years what topping up the leak! Since november last year when i bought it i have hardly ran it as i want to sort out a few things first.
There is no sign whatsoever of over heating, there is a little light coloured smoke on firing her up but none visible in rear view mirror when driving.
I plan to bleed the cooling system soon remove the rear heater and refill, should i do this first and then check the coolant for the dark stuff? Or will running the van with a possible head gasket problem lead to meltdown!
The van is going for a timing belt change in a few weeks should i ask the garage to have a look at the HG? I'd ideally like to learn about my vans workings inside out and try the job myself but if honest i'm a bit intimidated

Any help or advice on this matter would be greatly appreciated, again thanks to all on here!
