First off...yes this is the WRONG way to do things.

You won't be able to do it in a single field, but you can do it with multiple fields (i think):

Using your code sample, try the following for each type of field you want:

SELECT

DataType,

MAX(

CASE WHEN DataType in ('CURRENCY','NUMBER') THEN CAST(Value AS DECIMAL(18,2))

ELSE NULL END) AS MaxNumber,

MAX(

CASE WHEN DataType in ('Date') THEN CAST(Value AS Datetime)

ELSE NULL END) AS MaxDate,

FROM GetMaxValues

GROUP BY DataType

Hope this works!