Take a look at this solution that's being developed and manufactured by a member/enthusiast
If you *must* stay with oem audio, then this will be a great/promising solution
If this isn't/won't be for you, then looking at your list of wants/requirements, what's your definition of affordable? Reliability will be a factor, so I'd say stay away from the unbranded electronics you see on ebay/amazon offering these or similar functions.
You mention 'high quality sound' but this is going to be a moot point really if you intend to keep the OEM/DSP audio modules/speakers. Reason is the consensus for 'Sound Quality' from the OEM setup is 'mediocre' to 'ok' with many also saying it's 'poor' or 'rubbish'. There's a 'few' who say it's 'good' or great'. Look at products from GROM would be my suggestion, they do have 'phone calls' via bluetooth which you don't want, but the vast majority of E39 owners do want, so you can just not use that function if you wish as I don't think there's a specific, excellent reliability and quality product for you.
I see from your other thread you've retrofitted the OEM modules after there was one of the low quality chinese units in the car. Fair enough, but do realise/know that the older modules now are failing more often and each module has multiple points of failure and, natually, they're all relatively expensive to fix/resolve. You can get upgrades where possible, and they're relatively expensive too. The OEM DSP amplifier has had more reports over the last couple years of failing and I only know of 2 places that overhaul/repair them where possible and they're in Romania and Australia. If you don't mind keeping the OEM setup going along with those costs, no problem. However, much better value for money spent Vs modern features and functionality, reliability and a significant increase in sound quality is to go with a premium brand aftermarket shallow depth head unit. Some recent posts from members show these with an OEM looking fascia/fitting kit to give a very close to OEM look in the dash area! By upgrading the cabin speakers, adding a good 5 channel amplifier and a modest 8/10 inch subwoofer all installed well, will give you a fantastic increase in Sound Quality, performance and reliability and would be the route I'd recommend.
2003 Ex-Police E39 530 Diesel Touring. Alpine White, 260,000+ Miles & climbing!
19" Hartge Nova 5 - with custom painted 'Flash Chrome' centres & Polished Lips (set of 5)
19" Hartge Nova 5 - Awaiting custom re-finish (set of 4)
19" Hartge Classic - with custom powder coated centres in shadow chrome & diamond cut/no lacquer lips (set of 5)
20" Hartge Classic - Awaiting custom re-finish
19" 3Pc Forged wheels - ID retained until re-built & finished (2 sets)
20" 3Pc Forged wheels -ID retained until re-built & finished
19" Centres - ID retained until rebuilt & finished
Full Custom Audio install - Arc Audio KS Mini ampliifers. DLS 5.25" 2 way front components & 4" 2 way co-axials. JL Audio 8W3-V3 subwoofer in stealth enclosure. Parrot Asteroid Smart head unit, custom door builds for DLS front stage & Earthquake SWS6 flatline subwoofers, custom rear pillar speaker pods. LumiDock iPod dock,
H&R Springs, ExoticsTuning front lip spoiler, Sport F/R bumpers upgrade, Carbon Fibre effect printed Vinyl roof wrap, High gloss black vinyl window trim wrap, Hartge bonnet/hood & tailgate badges, rear rooftip spoiler, matt Black kidney grille, projector foglamps.
Hartge auto gearlever, Hartge Handbrake handle, Hartge door sill strips, E46 M3 CSL Alcantara/Tri-stitch SMG steering wheel, full Black Leather Sport seats,
Custom ECU re-map biased for more Power & Torque in Steptronic mode, EGR delete, K&N induction kit, Stainless steel de-cat, Stainless steel exhaust backbox with twin oval pipes. Manual 5 spd gearbox conversion.
Plenty more modifications to come!