You can certainly retrieve this information using the Marketing Cloud Fuel API, but at this time, you can only POST and PUT into Data Extensions using the REST API (refer to the Data Events methods), so you will have to use the SOAP API for now.
Here's a typical SOAP request envelope to retrieve data from a Data Extension:
<?xml version="1.0" encoding="utf-8"?>
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<soapenv:Header>
<fueloauth xmlns="http://exacttarget.com">InsertOAuthAccessTokenHere</fueloauth>
</soapenv:Header>
<soapenv:Body>
<RetrieveRequestMsg xmlns="http://exacttarget.com/wsdl/partnerAPI">
<RetrieveRequest>
<ObjectType>DataExtensionObject[InsertDataExtensionExternalKeyHere]</ObjectType>
<Properties>firstname</Properties>
<Properties>lastname</Properties>
<Properties>email</Properties>
<Filter xsi:type="SimpleFilterPart">
<Property>memberno</Property>
<SimpleOperator>equals</SimpleOperator>
<Value>123456</Value>
</Filter>
</RetrieveRequest>
</RetrieveRequestMsg>
</soapenv:Body>
</soapenv:Envelope>
Regarding tracking data (click, open, etc), you will need to create a Query Activity that uses a Data View, then you can create an Automation to run the Query Activity at a regular schedule.
Regarding your "50,000 allotted API requests", this is a soft-cap — you can make more requests than 50,000. There are no physical restrictions in place, although Marketing Cloud could implement them if the number of requests are too excessive.