If you want to have a go at electro mechanical engineering, fill your boots pal! Some of us resent being quoted stupid money, ie £11,000 for something I can partially do myself and then have checked by an expert, for £4000. I guess you're the sort of guy that only buys a new car just in case some unqualified person has changed the oil! To fully answer your question, I did fit my fire, then I had a qualified person come in and do the pipe work to it do I knew it was safe.
if you're an electrical mechanical engineer, then IMO this would mean that you ought to be reasonably competent to carry out the work you've described, then get someone experienced in this field to just commission it - though be prepared for additional costs if you have ballsed something up that needs to be sorted out to get it to the proper standards.
I've done similar for others that I've judged to be competent on a couple of occasions, but tbh I don't really like it as there are techniques we've perfected over the years for eg ensuring the brackets and framework doesn't either knock against the tiles, raise the tiles up, ensuring the rails and panels are all plumb level etc. and it'd be a right pain in the backside to find this has been done wrong and the entire array has to come off so we can correct it.
Both jobs I can remember doing in this way, there were issues with that we had to rectify, or the extra guy helping actually slowed our lads down because of them having to explain why we did things a certain way, instead of just getting on and doing it - not too serious, but it made me quite wary of working in this way in future even if they ought to be competent.
It takes a 3 man team about a day to carry out most installs, sometimes 2 including commissioning and scaf in winter if it's more complex - this isn't a particularly large staffing cost, and is probably offset by the vat being charged at 5% vs 20% for the equipment alone, so I doubt it's worth it unless you're vat registered.
eta - when confronted by this situation now, I usually just offer to do it at our trade rate if I view them as being trade customers, and allow them to assist with moving panels around for us on the ground etc if they want to help out. Even if it's just a one off job, this approach tends to lead to further referrals anyway, as people in a trade tend to spread the word about good workmanship and helpful attitudes etc.
