Consider a database(MSSQL 2005) that consists of 100+ tables which have primary keys defined to a certain degree. There are 'relationships' between tables, however these are not enforced with foreign key constraints.

Consider the following simplified example of typical types of tables I am dealing with. The are clear relations between the User and City and Province tables. However, they key issues is the inconsistent data types in the tables and naming conventions.

User:
    UserRowId [int] PK
    Name [varchar(50)]
    CityId [smallint]
    ProvinceRowId [bigint]

City:
    CityRowId [bigint] PK
    CityDescription [varchar(100)]

Province:
    ProvinceId [int] PK
    ProvinceDesc [varchar(50)]

I am considering a rewrite of the application (in ASP.net MVC) that uses this data source as is similar in design to MVC storefront. However I am going through a proof of concept phase and this is one of the stumbling blocks I have come across.

1. What are my options in terms of ORM choice that can be easily used and why?

2. Should I even be considering an ORM? (The reason I ask this is that most explanations and tutorials all work with relatively cleanly designed existing databases, or newly created ones when compared to mine. I am thus having a very hard time trying to find a way forward with this problem)

3. There is a huge amount of existing SQL queries, would a datamappper(eg IBatis.net) be more suitable since we could easily modify them to work and reuse the investment already made?

I have found this question on SO which indicates to me that an ORM can be used - however I get the impression that this a question of mapping?

Note: at the moment, the object model is not clearly defined as it was non-existent. The existing system pretty much did almost everything in SQL or consisted of overly complicated, and numerous queries to complete fucntionality. I am pretty much a noob and have zero experience around ORMs and MVC - so this an awesome learning curve I am on.