That's good advice from Nick.
I work for Elite Remaps, and on the Accord club I've also done the same with some members who want to go the DIY route. To be honest, the easiest thing to do is to sell the customer the map for their car and let them DIY flash it.
Something like the Galletto 1260 can do a variety of cars, and in addition to my Byteshooter and Kess V2 I keep the Galletto as a good backup tool - it's actually helped out a few times this year with customer cars, and it costs a few quid on ebay.
As Nick has described, you absolutely need to understand each map, and also understand the inter relatonship between each map. Engine tuning is not about software tuning, you need to understand how an engine works to become a good engine tuner!!
And Diesel tuning is really very different to petrol tuning. Remember, diesel is a lubricant as well as a fuel.
I've also seen some really bad remaps where we have corrected the sotware - in fact just this week I've tuned a 2006 A4 170bhp with a DPF that actually had maps missing, and the so called tuner had tried to run a DPF delete on it and ended up deleting maps in the process
It made me chuckle when I read the OP's comments about thinking the worst he could do was damage the ECU - in fact far from it, the EDC15 and 16 ECU's are totally recoverable, it's near impossible to cause them 'damage' in this way. But each component you try and tune is at risk if you do not know what you are doing...
Leave it to the professionals mate, a lot of time and investment has gone into perfecting the maps we provide.