Error: 'Operand data type varchar is invalid for sum operator' in SQL Server

I try this query

Select 
S.Name,S.No,
SUM(Case when s.Model='Cultus' then total else 0 end) as Cultus,
SUM(Case when s.Model ='vigo' then total else 0 end) as vigo,
SUM(total) total_v ,
 s.MA,MAX(S.Speed) Speed
 from (
Select
 RVU.Name,RVU.No,VV.Model,count(VV.Model) as total, RVU.MA as MA,RVU.Speed 
from 
 VV 
inner join RVU
on VV.MID=RVU.ID 
inner join RU on RVU.RID= RU.RID
    WHERE 
RU.StartDate>= '2016-04-01 00:00:00' and
RU.EndDate<='2016-04-30 23:59:59'   and
RU.Region= 'Paris'
and RVU.No= '651' AND Model <> ''
Group By RVU.Name,RVU.RegNo,VV.Model,RVU.MA,RVU.Speed ) S
GROUP BY
s.RegNo,s.Name,S.MA

Ouput

Name    No  Cultus  vigo    total_v   MA       Speed
David   651    2    0        2         1048      124
David   651    3    0        3         597      345
David   651    1    0        1         606     101
David   651    3    2        5         992     110

when i try this

Sum(MA) MA,

THIS SHOW ERROR

Operand data type varchar is invalid for sum operator.

I want result like this

Name    No     Cultus   vigo    total_v   MA    Speed
David   651    9         2       11       1799    345

when i try this

SUM(CASE ISNULL(MA,'') WHEN '' THEN 0 ELSE CAST(MA AS INT) END),

and also this

sum(cast (MA as int))

then output is

 Name   No  Cultus  vigo    total_v     MA     Speed
David   651    2    0        2          4192     124
David   651    3    0        3         2388      345
David   651    1    0        1         2424      101
David   651    3    2        5         4960      110

when i execute only this part this show multiple records with same data

 Select
     RVU.Name,RVU.No,VV.Model,count(VV.Model) as total, RVU.MA as MA,RVU.Speed 
    from 
     VV 
    inner join RVU
    on VV.MID=RVU.ID 
    inner join RU on RVU.RID= RU.RID
        WHERE 
    RU.StartDate>= '2016-04-01 00:00:00' and
    RU.EndDate<='2016-04-30 23:59:59'   and
    RU.Region= 'Paris'
    and RVU.No= '651' AND Model <> ''
    Group By RVU.Name,RVU.RegNo,VV.Model,RVU.MA,RVU.Speed

looks like value totally change