I would highly recommend reading through recent threads on COTH on the topic of customs. There are more than a few threads regarding horror stories.
My advice is don't do it. Find a saddle that fits off the rack if you possibly can. The only circumstance under which it is worth it is if the company offers you a 100%, no questions asked, money back guarantee if it doesn't work for you and your horse.
Some will only take the saddle back if it's a standard model not one that you customize with a different flap length, flap angle or other such feature. There are also plenty of threads where people ordered a custom saddle that didn't end up fitting their horse but the company said it did and then it became a battle back and forth. I was in that situation once and once was enough. Never again!!!
On a positive note, I went custom - there was nothing used or on the demo lists that fit both horse & rider - new saddle is lovely, horse & rider are happy, rep is fantastic, company has been very considerate.
If you can find an existing saddle that fits, buy it
If that's not possible, use the custom = disapointment stories on forums as an educational tool: learn about saddle fit, read the fine print, ask questions, insist on multiple demo rides, video the saddle trials so you can compare what you feel with what you see, get your trainer/coach involved in the process, have more than 1 trainer involved, try saddles from multiple companies/fitters/reps ...
What do you mean by "weird bump on the bottomish of his withers"? Is this a muscle knot or a subluxed vertebrae?
Look for a saddle fitter with credentials -- after a few bad experiences I will only use a fitter who is trained by the Society of Master Saddlers (British credentials). S/He will have a variety of saddles (used as well) that you could try out, and most likely a custom option.
I have a custom Hastilow Concept which I love and can be adjusted to fit my horse no matter how much his body changes over his lifetime (or to fit a different horse).