Do it yourself if you've got some tools, either get a haynes repair manual (from Repco) youtube links helpful as well.
(dohc)
(sohc)Use factory genuine parts its not worth the risk taking shortcuts, or penny pinching at the risk blowing a motor particularly if its an interferance one ie dohc/turbo and later engines eg
[ had an 86 leone did 200k belt idlers tensioner etc but sprocket idler reused with new bearing pressed by subaru dealer and that failed 1 year later ie seized destroying 1 of the 2 timing belts ,luckily old ea ohc enine non interferance so no serious damage just inconvenience so moral of story use new parts]
its a good idea to mark everything with white marker like twink marker, ie line up and mark all the timing marks on cam pulleys, crank sprocket with marks on timing covers and mark old belt and have everything in the correct postion BEFORE disassembly. need to replace :
1 belt obviously
2 all idlers (plain and sprocketed { toothed}) and tensioner/idler- not really a good idea to reuse the old one.
3 oil seals on crankshaft and cam shafts
4 water pump at 200k or 8 years not necesarry at 100k but if car 10year + probably a good idea to replace.
5 Some suggest replacing the oil pump not sure about this but l'm of the opinion that it should be ok if there has been regular servicing.
the biggest killer of subaru engines (cracked heads, blown head gaskets, cooked motors, clogged radiators) is lack of servicing ie regular oil/filter changes and
replacement of coolant / flushing of cooling system (every 2 years)
because subaru engine is in a flat boxer configuration the oil and water (coolant) stays in contact with gaskets and oil / water in bad condition can eat away at gaskets, clogg radiators etc. eg l had an 86 leone with a radiator 85% clogged after unexplained overheating driving uphill, had a flow test done, had to be taken apart and physically "rodded out". so managed to save it. no thanks to the previous owners.
sorry have gone on a bit but hope this is useful, on sub no. 5 and for 20 years :}