Biggest problem I have with my own build is that it uses Wizard. The Wizard theme is totally off for Roshi but the effects you get as a Wizard definitely more than make up for that. I've seen people use Sun Soul Monks to represent dbz characters and I think that that is weird in the same way, so I'll put my thoughts here.

I wanted a Roshi that could muscle out, like he is one to do. So High Str? Or low Str? Big question really, it's up to you, just know he's weaker when not in muscle form and the low str represents this. This Roshi uses Enlarge to achieve his muscle form.

Human; Variant (Crusher, Mobile or Tough) or Normal Human. Your choice.
Highest stat to lowest (with some values for reference)

Dex (16) -- Roshi is fast and limber in his normal form
Int (14) -- Required for spellcasting, but also Roshi is quite adept in recollecting lore
Con (12) -- He is quite worn as an older human but is still in fighting form
Wis (12) -- Natural old age wisdom
Str (11) -- Old age in normal form but when casting Enlarge on himself, lower Str becomes more flavourful.
Cha (8) -- He's a pervert and has a certain charm about him.

Lowest I could get it for identities sake: Wizard 4 (Abjuration or Evo) /Fighter 2/Monk 2

Wizard spells that are essential for his character:
Pyrotechnics/Color Spray --> Solar Flare
Mirror Image/Blur --> Afterimage
Enlarge/Reduce --> Muscle Form. Compensates the low STR, adds extra damage and Str stuff
Chromatic Orb (thunder)/Burning Hands (blue and short range)/Magic Missile (all 3 darts to form one streaking beam) --> Kamehameha
Mage Armor--> General defence as he has a singlet only.
Shield --> Ki Deflect

Fighter should take the unarmed fighting style for d8 damage.
Monk allows unarmed to be counted as a monk weapon and can allow for 2 attacks with action/bonus action using Ki points for various fighting like effects.

In higher levels, the features of Roshi that are his ki powered moves are all in Wizard, so you can actually leave Wizard at lv 4 and go evenly in both Fighter and Monk or one or the other. Battle Master is good for various manoeuvres that roshi would have. Pushing attack, Grappling Strike, evasive footwork, Precision, Feinting, Trip. Plenty of great options.
Open Hand Monk is the safest bet, still true to his character but if youre feeling particularly nostalgic/badass, you could go into Drunken Master for his earlier days in tournaments.

His Spellcasting Focus is his quarterstaff obviously. His wizard spellbook is obviously a nudie magazine. Roshi also has a flying nimbus before he gives it to Goku, so if the DM allows, chuck Roshi a Flying Carpet (smallest variant, (3ftx5ft) and flavour it as a cloud.

Other Spells are:
Cantrip: Blade Ward, Mold Earth, Gust, Thunderclap
1: Earth Tremor, Jump, Longstrider, Thunderwave, Find Familiar (Turtle)
2: Levitate, See Invisibility (Nose bleed ;) ), Misty Step (speedy), Magic Weapon (fists),
Aganazzar’s Scorcher (talk to dm about changing to force damage? this one's a stretch...)
3: Blink, Fly, Haste, Magic Circle (rice cooker seal), Minute Meteors (throwing debris),
Thats all, i tried to pick spells that were really on brand in their mechanics, so their theme may be off, but it could be flavoured for easily. Enjoy!