Its a case of personal choice really..... Bit like decorating. You can pay to get the professionals in or do it yourself. Getting the professionals in means you should get a better finish, no need to pop off to B&Q to get something you forgot 30mins after your last trip there, no need for you to find the time to do it and finally some people just don't want to diy.
Doing It Yourself is a lot cheeper but you still need your wits about you because if you wire up a flow control, pump or split charger wrongly then its you who has to pay for a new replacement and wait for it to be delivered.
So DIY can save you money but pro fitting can save time and the hassel.
Regards
Mr H