We maintain about 50 brands within our company and I would like to have a standard email template where the brand logo would swap out AND the body of the email (not the header and footer) would be inserted at send. The problem I have had is that I CANNOT get the body to work with the controller. I want to be able to create a template and reference any field  like {!opp.companyname__c}_logo,jpg for example. Right now I am using objects to retrieve the parts but this doesnt connect it with the controller. How can I approch this in a better way?

Code:

public class oppSendMail{
    public string subject { get; set; }
    public string body { get ; set; }
    private final Opportunity oppo { get ; set; }
    private final Account acc;
    private final OpportunityEmailHtml__c objHTML;
    private final Emails_Subject_And_Body__c objESB;
    private string webPartner {get; set; }
    
    public oppSendMail(){
           oppo=[select ID,Name,etc];

        objHTML = [select EmailHtml__c from OpportunityEmailHtml__c where Name = : 'GlobalET'];
        
        objESB = [select Body__c, Subject__c from Emails_Subject_And_Body__c where Name =: 'Hold' Limit 1];
        
        subject = String.valueOf(objESB.get('Subject__c'));
        body = String.valueOf(objHTML.get('EmailHtml__c'));
        webPartner = String.valueOf(oppo.get('WebPartner__c'));
        body = body.Replace('{!OppBasic.WebPartner__c}', webPartner);
        body = body.Replace('@Message', String.valueOf(objESB.get('Body__c')));
    }
    
    public Opportunity getOppBasic(){
        return oppo;
    }
    
    public OpportunityEmailHtml__c getETBasic(){
        return objHTML;
    }
    
    public PageReference send() {
        // Define the email
        Messaging.SingleEmailMessage email = new Messaging.SingleEmailMessage();
        String addresses;
        addresses = String.valueOf(oppo.get('email__c'));
        String[] toAddresses = addresses.split(':', 0);
        // Sets the paramaters of the email
        email.setSubject( subject );
        email.setToAddresses( toAddresses );
        email.setHtmlBody(body);
        // Sends the email
        Messaging.SendEmailResult [] r =
        Messaging.sendEmail(new Messaging.SingleEmailMessage[] {email});
        
        PageReference objPage = new PageReference(Apexpages.currentPage().getParameters().get('retURL'));
        objPage.setRedirect(true);
        return objPage;
    }
}