mishar

Not necessarily. Guy here had problem with ripped timing belt. His trusted mechanic probably forced it. There is "professional" on YouTube doing it all wrong.

dvs_dave

Of course, and with everything, there are instances out there but they are anecdotal rather than representative. If it's a shop you trust that has experience doing the job and uses quality parts, then there is no reason to presume the worst and think that they will do it wrong. There is much more chance of a first timer getting something wrong.

But like I said, I'm not telling anyone how to spend their money, but the peace of mind, time saved, and 1yr/12k parts/labor warranty that comes with having a shop do it is worth it to me for such an important job.

MP4.2+6.0

A few observations:

1. Some of the parts are really hard to figure out either who the OES actually is or to go by better known brand names. Thus with a belt from a Continental or a Gates you can do that, or a bearing from an SK/SKF/SKS or a gasket or seal from a Reinz, or a thermostat from a Behr, but in other cases not really.
2. To the prior point, I used a serpentine belt tensioner at one point on my 2000 C5 4.2 from NTN. They come up in the kit suppliers from the link. At the time I got it, I thought it was Euro quality or quasi OES. But when I bolted it on, I noticed it seemed a little misaligned in terms of squareness to motor face. Next time I had belt off and saw the same thing after plenty of run in, yet another piece of metal into the recycle bin. Went back to dealer part; wasted money.
3. Some of the parts in the kit are sort of over the top to me. Like the cam seals. Yeah, I know some of the kits have them. Good luck though. Having disassembled a 4.2 head once on the bench and having fully pulled the cams and cam drive another time with the head still on the block, I think you pretty much have to have the cams out to have it make sense to change those seals. Would be very hard to do with a seal puller, and would involve a bunch of other parts removal; two are less accessible and two are somewhat easier. I don't tend to mess with main crank seal either if it isn't weeping/leaking.
4. I know some have specific views on metal vs. plastic water pump impellers. I own a W12 and have an OE dealer water pump sitting in a VW/Audi branded box. Clearly a plastic impeller. So, will my motor blow up? Should I send it back? Should I (laughably) machine my own and try to bond it on? Should I try to find an aftermarket one that's metal? Should I just leave my old one on (whatever it is) since I can only get "plastic" in a current Audi/VW box. No, of course not; it is what it is as OE. I've seen many a metal impeller over the years that is a cheap stamped steel design too that likely has poor flow characteristics. Somehow on my "plastic" W12 one meantime, I don't think Audi/VW tried to cost reduce the water pump to the cheapest thing on the planet; the fitment is W12 and W8. For that matter, most all my water pump failures--have been at the seal, not a loose or deteriorated impeller. Rather than focus too much on that, from many--too many--an experience with water pumps on Audis in my younger and cheaper days, it is an area where I now only use dealer supplied. On a 4.2 specifically, either a failed water pump or thermostat is a nightmare that then forces the whole timing belt job, so to me among a lot of the debatable parts these two are easy ones to decide to replace preemptively.

Net, having been burned enough times now, on core parts I get dealer or OES. If it is a known brand like Gates, Continental, the bearing guys, Reinz, etc. I will gladly take some cost out with theirs. But a number of parts are shots in the dark where the OES's really aren't known (even water pump); there I go back to (discounted) dealer. Audi USA also reduced parts prices for higher volume "wear items" some years ago. Thus some parts now aren't so ridiculously priced--belts among them IIRC--though others sometimes are quite pricy.

Specific to vendors if it helps, if it is "dealer" then for me it is audiusapart.com, aka Sunset Audi in Portland OR. If I am looking for OES alternates, yes Europa perhaps. FWIW, my usual choice if they have it in OES or known quality branded for pricing, cheaper ship, etc. is autohausaz and sometimes rockauto. Sometimes ecstuning, which has a nice site and clear pictures, but is often close to dealer pricing and their shipping is not terribly competitive. They also have smaller and larger (core only vs. extended) kits IIRC.