This a much bigger question than first appears. If we limit it to what is a cost effective DIY solution to "read my Fiat and simple maintenance issues eg change the oil in my euro 6 or add a new MAF sensor" then the answer for me reduces to
Hardware
Windows laptop
Software : multiecuscan
Alfaobd for Windows.
Android phone, tablet and alfaobd

multiecuscan.net is also available for IOS but with an annual subscription of £50 and other factors a number of apple users buy a windows laptop specificaly.

Specific obd2 readers are also needed as as not all talk Fiat.
Gendan is a distributor for both mes and USB dongle .
For Bluetooth I use a vgate icar pro from ebay £25 they work, cheap £10 ones do not.
To use other ecus you may need various coloured adapters. A yellow adapter is sufficient for can based Ducatos.
I use a win10 multiecuscan with both obdsx USB dongle and vgate..
Alfaobd Samsung j6 phone a8 or tabA tablet and vgate or obdsx and a USB convertor cable. ie this can be achieved a number of ways.

Generic ones will not do many of the 2 way applications eg turning lights or equipment on to show they are under control, these 2 are the only diy product that can reset the oil degradation counter. They can interogate the systems in depth eg give you the software version you are running.
Buy a £10 device you get £10 of info when you want to do more there is the advice here on how to use them , almost dealer spec for £100 (MES +win laptop) to less than £60(alfa obd android device)

Interfaces
As an update I have since aquired a cheap usb interface from Ebay. The key point is for the Ebay/ other description to include the words "suitable for multiecuscan". I include a photo of the advert i used and the device that actually arrived. Note the colour and wording difference. Did it work for less than £15 : yes it does. I ran a series of tests including datasaving using one of my templates, resetting the oil degradation algorithym and 2 way coms turning warning lights on and off. I just pluged into my van and Laptop with a working copy of MES, so I did not use the supplied driver CD.
I have only mentioned the 3 obd readers I use, there are many others suitable. If you have found good alternatives (ones that work) please add them in the discussion section. I can only recommend purchase from the Multiecuscan distributor Gendan from EBAY or Amazon it must be Caveat emptor (let the buyer beware).


obdlink.png


elm-adaptor.jpg

elm-adaptor.png

SGW security gateway
If you have SGW with a security gateway there is a cable that can be used to bypass the gateway. I can offer no advice if a particular model has one as it was introduced on different models at different times (Starting 2018).
The follwing is from a thread I posted on SGW implementation
I searched a variety of fora for threads that included SGW and came up with the table bellow. This is data collected from owner statements. The main issue of veracity is based around the date, is it the year of manufacture or registration, motorhomes may be registered 6 to 18 months after manufacture. Also there is Regional variation eg the following 2 statements are french in origion:
SGW appeared on 24/02/2019 on Series 6 (Fiat code 6Y, nuovo ducato euro6d). 2.3 Adblu (the modification number 23582, changes the glove box drawer.) (In UK this model didn't surface until sept 19.)
Ducato (Series 7c, ducato FCA 2021), SGW is integrated to BCM

DucatoPeugeotCitroenyear/reg plateSGWSGW in BCM
Yes19NoNo
Yes20Yes2.3 Adbu
Yes21YesYes
Yes21YesYesBox behind glovebox
Yes21YesYesBox behind glovebox
Yes22No
Yes71Yes
Yes20No
Yes21No
Yes21No
Yes21No
Yes22Yes
YesYes


Which is best Multiecuscan or AlfaOBD?
You tell me, Both cover the things I need, there are minor differences.
If I had a Dodge Ram then the Support for Alfobd is good but I have a Ducato.

For Me I am more comfortable with Multiecuscan and there is better support here and Multiecuscan.net
I also prefer the greater number of PIDs that can be monitored at a time. (i also don't hen (change sex of bird) up saving the data on MES whereas i do with Alfaobd (could just be a matter of familiarity)).
I have both so am happy to know I have some capability in the van at all times as long as i don't forget my phone.