Actually the E-Plus network doesn't provide the best coverage for Germany, but it's still good and since E-Plus and O2 (both used only the GSM1800 band before) got EGSM900 frequencies last year, they're about to close the gaps quickly with futher reaching EGSM900 basestations. However E-Plus' 3G network is cappy - it has bad coverage, is overloaded and doesn't support HSPA at all.
Regarding the porting process you should know the following:
1.) If you're on a postpaid plan you need to cancel the contract before initiating the porting or if you're a prepaid customer you need to send a so-called "Verzichtserklärung" (disclaimer) to your provider, so they'll release your number for porting. You can download a corresponding form for T-Mobile from
http://klarmobil.de/nl/verzicht/tmobile.pdf
2.) The old provider will always charge about € 25 for the export of your existing number. If you're a prepaid customer you must make sure you have at least € 25 of credit left. Otherwise the old provider will refuse the porting.
3.) You shouldn't order your new SIM card 2-3 days before you have send in the "Verzichtserklärung".